The "opinion rule" provides that "evidence of an opinion" is generally inadmissible (Evidence Act 2008 s76).
While the Evidence Act 2008 does not define the term "opinion", it has been held that it refers to an "inference drawn from observed and communicable data" (Allstate Life Insurance Co v ANZ Banking Group Ltd (No 5) (1996) 64 FCR 73) or "evidence of a conclusion, usually judgmental or debatable, reasoned from facts" (R. W. Miller v Krupp Australia Pty Ltd (1992) 34 NSWLR 129. See also Hodgson v Amcor Ltd  VSC 272).
Evidence of an "opinion" can be distinguished from evidence of a "fact". Where a witness simply gives evidence of something he or she observed, or of a particular state of past or present affairs, that will not be opinion evidence (see, e.g., Bodney v Bennell (2008) 167 FCR 84; Australian Securities and Investments Commission v Vines  NSWSC 1095; Hodgson v Amcor Ltd  VSC 272).
Evidence of an "opinion" can also be distinguished from evidence of "experience". Evidence of a witness’ experiences will not be opinion evidence, unless the witness proceeds to draw some inference from those experiences (see, e.g., Clark v Ryan (1960) 103 CLR 486).
Hearsay evidence of an opinion is itself opinion evidence (R v Whyte  NSWCCA 75; Jackson v Lithgow City Council  NSWCA 312. But cf Australian Securities and Investments Commission v Rich (2005) 216 ALR 320).
The following matters have been held not to be opinion evidence for the purposes of the Uniform Evidence Acts:
Evidence given by anthropologists of their observations (Bodney v Bennell (2008) 167 FCR 84).
Evidence given by expert witnesses regarding their observations of an attempted reconstruction of an accident (Collaroy Services Beach Club Ltd v Haywood  NSWCA 21).
Evidence given by witnesses about what they would have done in a hypothetical situation (Allstate Life Insurance Co v ANZ Banking Group Ltd (No 5) (1996) 64 FCR 73; Hughes Aircraft Systems International v Airservices Australia (1997) 80 FCR 276; Seltsam Pty Ltd v McNeill  NSWCA 158).
Evidence given by an expert witness about how a certain type of professional would generally act in particular circumstances (Australian Securities and Investments Commission v Vines  NSWSC 1095).
Evidence given by a member of an organisation about the information available to that organisation (Bank of Valletta PLC v National Crime Authority (1999) 90 FCR 565).
Evidence describing the workings of a complex piece of equipment, such as a computer (Hodgson v Amcor Ltd  VSC 272).
It appears that identification evidence may be either evidence of fact or evidence of opinion, depending on the circumstances:
Where there is little risk of misidentification (e.g., where the witness identifies a person, clearly depicted in a studio photograph, as his or her spouse), identification evidence will normally be regarded as a statement of fact;
Where a real risk of misidentification is present (e.g., where the identification is made from a photograph which does not clearly depict the person who is its subject), it will usually be appropriate to classify identification evidence as opinion evidence (see, e.g., R v Leung (1999) 47 NSWLR 405; Smith v R (2001) 206 CLR 650 per Kirby J; R v Drollett  NSWCCA 356; R v Marsh  NSWCCA 331).
This means that, in every case involving identification evidence, the trial judge must examine the nature of the evidence proposed to be adduced, and all of the relevant circumstances, to determine whether the evidence is opinion evidence (and is thus subject to the admissibility provisions outlined below) (R v Drollett  NSWCCA 356).
Evidence in the Form of an Opinion
The opinion rule refers to "evidence of an opinion". It has been suggested that this phrase covers both of the following:
Evidence which is substantively an opinion (i.e. an inference drawn from observed and communicable data); and
Evidence of a fact or observation which is given in the form of an opinion (e.g., evidence from a witness that "in his or her opinion" another person was intoxicated) (S Odgers, Uniform Evidence Law (8th ed, 2009) [1.3.4090]).
Admissibility of Opinion Evidence
The opinion rule states that evidence of an opinion is not admissible to prove the subject matter of the opinion (Evidence Act 2008 s76).
There are two main exceptions to the opinion rule:
Opinion evidence from an "expert" witness is admissible if it is wholly or substantially based on specialised knowledge that the witness has obtained from training, study or experience (Evidence Act 2008 s79).
Opinion evidence from a "lay" witness is admissible if it is based on what the witness saw, heard or otherwise perceived about a matter or event, and evidence of the opinion is necessary to obtain an adequate account or understanding of the witness’ perception of the matter or event (Evidence Act 2008 s78).
Other exceptions to the opinion rule include:
Summaries of voluminous or complex documents (s50(3));
Evidence that is admitted to prove something other than the subject matter of the opinion (s77);
Evidence about the existence or content of Aboriginal and Torres Strait Islander traditional laws and customs (s78A);
Evidence of an admission (s81);
Evidence of matters which are exceptions to the rule that usually excludes evidence of judgments and convictions (s92(3)); and
Opinion evidence, like any other evidence, is only admissible if it is relevant (Evidence Act 2008 ss55-56).
The relevance of opinion evidence may depend upon an assessment of its factual basis. Opinion evidence will not be relevant if it does not have a rational factual basis (Evidence Act 2008 s55; R v Panetta (1997) 26 MVR 332).
Abolition of Common Knowledge and Ultimate Issue Rules
Opinion evidence may not be excluded simply on the basis that it is about:
A matter of common knowledge; or
An ultimate issue in the proceeding (Evidence Act 2008 s80).
Although the effect of this provision is to abolish the common knowledge and ultimate issue rules, it should be noted that s80 does not make any evidence admissible. It simply provides that opinion evidence is not inadmissible on the specified bases (Idoport Pty Ltd v National Australia Bank Ltd (2000) 50 NSWLR 640).
This means that evidence which is about a matter of common knowledge, or which relates to the ultimate issue in a proceeding, should not always be admitted. Such evidence may be inadmissible on another basis (e.g., because it is not wholly or substantially based on specialised knowledge, and thus does not comply with the requirements of s79), or it may be excluded at the judge’s discretion under ss135 or 137 (see, e.g., R v GK (2001) 53 NSWLR 317; Idoport Pty Ltd v National Australia Bank Ltd (2000) 50 NSWLR 640; Yates Property Corp v Boland (1998) 85 FCR 84).
Expert Evidence in Homicide Cases Involving Family Violence
In homicide cases involving family violence, evidence may be led to show:
The cumulative effect of family violence on a person, including the psychological effect;
The social, cultural or economic factors that may affect a person affected by family violence;
The nature and dynamics of relationships affected by family violence, including the possible consequences of separation from the abuser;
The psychological effect of violence on people who are or who have been in a relationship affected by family violence; and
The social or economic factors that affect people who are or who have been in a relationship affected by family violence (Crimes Act 1958 s9AH).
Identification of Factual Basis
At common law, when expert opinion evidence is admitted:
The witness must state the factual basis for any conclusions he or she draws; and
The witness should explain how he or she has reached those conclusions (Clark v Ryan (1960) 103 CLR 486; R v Bonython (1984) 38 SASR 45; R v BDX (2009) 24 VR 288).
This allows the jury to determine whether the opinion has any value in light of their findings of fact, and to assess the weight to be given to that opinion (Arnotts Ltd v Trade Practices Commission (1990) 24 FCR 313; R v GK (2001) 53 NSWLR 317).
While the Evidence Act 2008 does not require witnesses to state the factual basis for their conclusions or to demonstrate their reasoning process, in the other Uniform Evidence Act jurisdictions they have generally been required to do so (see, e.g., Makita (Australia) Pty Ltd v Sprowles (2001) 52 NSWLR 705; Hevi Lift (PNG) Ltd v Etherington  NSWCA 42; Seven Network Ltd v News Ltd (No 15)  FCA 515).
Thus, it may remain prudent to require expert witnesses to identify the facts they have relied upon to form their opinion (see, e.g., Arnotts Ltd v Trade Practices Commission (1990) 24 FCR 313; R v GK (2001) 53 NSWLR 317; Makita (Australia) Pty Ltd v Sprowles (2001) 52 NSWLR 705. See also R v Anderson (2000) 1 VR 1; R v Ryan  VSCA 176; R v Johnson (1994) 75 A Crim R 522).
Experts should also be required to explain how their opinion was reached, and the application of their expertise (R v Johnson (1994) 75 A Crim R 522; R v Haidley & Alford (1984) VR 229).
Experts may consider matters of common knowledge, as well as their specialised knowledge, when forming their opinion. They should explain that they are relying on this accumulation of knowledge, as well as the way in which they acquired that knowledge (Velevski v R (2002) 187 ALR 233; Arnotts Ltd v Trade Practices Commission (1990) 24 FCR 313).
The need to identify and explain the factual basis of an expert opinion will only arise where that basis is contested. If evidence is adduced without objection, the trial judge may ordinarily assume that all matters crucial to the admissibility of the evidence are conceded by the opposing party (Sydneywide Distributors Pty Ltd v Red Bull Australia Pty Ltd (2002) 234 FCR 549;  FCAFC 157).
While the failure of an expert to explain his or her reasoning process will not necessarily render the evidence inadmissible, it will usually mean that it is less persuasive for the jury (HG v R (1999) 197 CLR 414 per Gaudron J. See also Guide Dog Owners’ & Friends’ Association Inc v Guide Dog Association (NSW & ACT) (1998) 154 ALR 527).
Hearsay Evidence Admitted as the Basis of an Opinion
Witnesses’ opinions will often be based on out-of-court representations. Evidence of those representations will generally be admissible to explain the assumptions on which an opinion is based (see, e.g., R v Lawson  NSWCCA 214).
If admitted for this purpose, an out-of-court representation may also be used to prove the existence of any facts asserted in that representation (Evidence Act 2008 s60).
It has been suggested that, due to the potential unfairness that may follow from the application of this rule, it may be preferable to avoid admitting evidence of out-of-court representations solely to explain the assumptions on which an opinion is based (R v Lawson  NSWCCA 214 per Sperling J).
If such evidence is admitted, it may be desirable to use the discretion in s136 to limit the use that may be made of such evidence (Roach v Page (No 11)  NSWSC 907. But see Harrington-Smith v Western Australia (No 2) (2003) 130 FCR 424; Alphapharm Pty Ltd v H Lundbeck A/S  FCA 559; Bodney v Bennell (2008) 167 FCR 84).
Use of Opinion Evidence
The precise way in which the jury may use opinion evidence will vary depending on the nature of the evidence given and the purpose for which it was admitted (see "Admissibility of Opinion Evidence" above).
This topic addresses the uses that may be made of opinion evidence that is admitted under Evidence Act 2008 s79 (expert opinion evidence), s78 (lay opinion evidence) and s77 (evidence admitted for a different purpose).
Expert Opinion Evidence
The jury may use expert opinion evidence:
To understand other evidence in the case; or
As the basis for drawing an inference (Taylor v R (1978) 22 ALR 599; Farrell v R (1998) 194 CLR 286).
Where an expert adopts an industry text or journal as an authoritative source, the jury may use any information contained in the text or journal as facts in the case if:
They accept that the expert has adopted the document; and
They accept the expert’s evidence about that document. (PQ v Australian Red Cross Society & Ors  VR 19).
Lay Opinion Evidence
Lay opinion evidence may help the jury to understand the matter or event about which the witness gave evidence (see, e.g., R v Leung (1999) 47 NSWLR 405).
Opinion Evidence Admitted for a Different Purpose
If evidence of an opinion is relevant and admissible for a purpose other than proving the existence of the fact which is the subject of the opinion, the exclusionary opinion rule (s76) does not apply (Evidence Act 2008 s77).
In such cases, the evidence may also be used to prove the existence of the fact which is the subject of the opinion (Allstate Life Insurance Co v ANZ Banking Group Ltd (No 5) (1996) 64 FCR 73; Hughes Aircraft Systems International v Airservices Australia (1997) 80 FCR 276).
In some cases it will be undesirable for the evidence to be used in this way. In such circumstances it may be appropriate to use the discretion in s136 to limit the use that may be made of such evidence (see, e.g., Murex Diagnostics Australia Pty Ltd v Chiron Corporation (1995) 62 FCR 424; Perpetual Trustee Company Ltd v George; Estate of Conacher (No 2) 28/11/97 NSW SC; Roach v Page (No 11)  NSWSC 907; James v Launceston City Council (2004) 13 Tas R 89;  TASSC 69).
The need for a direction on opinion evidence depends on whether a direction is sought or whether there are substantial and compelling reasons for giving a direction in the absence of a request (Jury Directions Act 2015 ss14 - 16). See Directions Under Jury Directions Act 2015 for information on when directions are required. The following sections describe the content of directions, if the judge gives directions on opinion evidence.
The content of the jury directions will depend on the nature of the evidence given and the ways in which the jury may use that evidence (see "Use of Opinion Evidence" above).
No guidance has yet been provided about the directions to be given in relation to lay opinion evidence. The remainder of this topic therefore only addresses directions to be given in relation to expert opinion evidence.
Directions About Expert Opinion Evidence
Expert opinion often involves unfamiliar and technical matters. Judicial directions should ensure that the jury can understand the evidence and apply it to the facts of the case (Velevski v R (2002) 187 ALR 233; R v Gemmill (2004) 8 VR 242).
The judge’s directions on opinion evidence should explain the significance of the evidence in the context of the case, and give any special directions needed to enable the jury to assess and use the evidence that has been led (Taylor v R (1978) 22 ALR 599).
The judge should ensure that the jury understands that, generally, only an expert is permitted to give evidence of his or her opinion. Other witnesses are generally limited to giving evidence of their own observations (Taylor v R (1978) 22 ALR 599; Farrell v R (1998) 194 CLR 286; Ramsay v Watson (1961) 108 CLR 642).
Where an expert witness has given evidence about the "ultimate issue", the judge should make it clear that it is the jury’s role to determine the issue (Australian Law Reform Commission, Evidence(Interim), Report 26 (1985) vol.1 para 743. See also Idoport Pty Ltd v National Australia Bank Ltd (2000) 50 NSWLR 640).
It is for the jury to decide whether an opinion is credible and what weight it should be given (R v Anderson (2000) 1 VR 1; Velevski v R (2002) 187 ALR 233).
The judge should tell the jury that they are entitled to reject the evidence if they are not satisfied that the science or the testing is sufficiently accurate, reliable or dependable. The jury is not bound by the opinion of experts, and must not be overawed by the scientific appearance of their opinions (R v Pantoja (1998) 88 A Crim R 554; R v Karger (2002) 83 SASR 135; United States v Baller (1975) 519 Fed 2d 463; R v Gilmore  2 NSWLR 935; R v Duke (1979) 22 SASR 46; R v Kotzmann  2 VR 123; R v Parker VicCA 10/8/1995).
Where a witness has been cross-examined about the nature and quality of his or her expertise, the jury should be told that a witness’ competency and credit are relevant matters to take into account when determining the weight to be given to his or her evidence (see, e.g., Polycarpou v Australian Wire Industries Pty Ltd (1995) 36 NSWLR 49).
The jury should be told that expert evidence is only as valuable as the facts supporting the evidence. That evidence only has probative value if the jury accepts the facts that form the basis of the evidence (R v Kotzmann  2 VR 123).
The jury should be directed to examine the basis on which the expert formed his or her opinion, and to determine whether the facts constituting the basis have been proven (Nguyen v R (2007) 173 A Crim R 557).
The judge should tell the jury to consider the following matters when they evaluate the evidence of experts:
The witness’ demeanour;
The way the opinion is expressed;
The quality of the reasons for the opinion;
The facts offered by the expert in support of his or her conclusions;
The witness’ response to cross-examination;
Whether the witness appeared to be impartial, or whether s/he was biased and overstated his or her evidence (R v Anderson (2000) 1 VR 1; Taylor v R (1978) 22 ALR 599).
Unanimous Expert Evidence
Where there is unanimous agreement amongst expert witnesses, the jury should be told that they are not bound by the experts’ opinion. However, they may only reject that evidence if:
The facts underlying the opinion are not present;
The process of reasoning leading to the opinion is unsound; or
There is some factor that casts doubt on the validity of the opinion expressed (Taylor v R (1978) 22 ALR 599; R v Matusevich & Thompson  VR 470; R v Matheson  1 WLR 474; R v Hilder (1997) 97 A Crim R 70; R v Klamo (2008) 18 VR 644).
Conflicting Expert Evidence
Where the evidence of expert witnesses conflicts, the jury should be told that they are entitled to prefer the evidence of one expert over another. The role of the jury is to select the evidence that they shall accept, and this includes expert evidence (Velevski v R (2002) 187 ALR 233; Chamberlain v R (No 2) (1983) 153 CLR 521; R v Gemmill (2004) 8 VR 242; R v GK (2001) 53 NSWLR 317; R v NCT (2009) 26 VR 247).
It is therefore not appropriate for the judge to direct the jury that one expert is superior to another. This intrudes on the role of the jury (R v Gemmill (2004) 8 VR 242).
In exceptional cases, the jury may be incapable of resolving a conflict between experts on matters of science. The judge should tell the jury that they must not accept disputed scientific evidence that is unfavourable to the accused unless there is a good reason to reject the defence evidence (Velevski v R (2002) 187 ALR 233).
Where the jury’s decision about which expert to accept is likely to determine the accused’s guilt or innocence, the judge may tell the jury that they may only accept the evidence of the expert who is adverse to the accused if they are satisfied beyond reasonable doubt that his or her opinion is correct (R v Anderson (2000) 1 VR 1; R v Sodo (1975) 61 Cr App R 131).
A direction that the jury must be satisfied beyond reasonable doubt that the prosecution expert is correct should not be given in all cases. Such a direction is only required when there are conflicting experts, and resolution of that conflict by the jury is likely to determine the case against the accused. In other cases the direction should not be given, as it will tend to isolate one piece of evidence and invite the jury to decide the case solely on the basis of that evidence (R v Middleton  WASCA 213; R v Anderson (2000) 1 VR 1; R v Nicholas  Tas R (NC) N24).
Expert Evidence About Witness Credibility or Reliability
Where expert evidence about a witness’ reliability is given, the judge should clearly direct the jury that they must not allow the expert to usurp their function of assessing the credibility of witnesses (Farrell v R (1998) 194 CLR 286).
If the evidence of the expert is limited to suggesting that the witness may suffer a disorder that affects his or her reliability, the judge may comment on the limited probative value of that evidence (Farrell v R (1998) 194 CLR 286).
Expert Evidence About Mental Impairment
Expert evidence will usually be necessary to establish a defence under s20 of the Crimes (Mental Impairment and Unfitness to be Tried) Act 1997.
In such cases, the judge should relate the expert evidence to the test for mental impairment (Taylor v R (1978) 22 ALR 599; Mizzi v R (1960) 105 CLR 659. See Mental Impairment).
While the jury is not bound by the expert evidence, the judge must ensure that they do not ignore unchallenged expert evidence and substitute their common sense view of the evidence (R v Wiese  VR 953; R v Gemmill (2004) 8 VR 242).
The judge should warn the jury that, when assessing the expert evidence, they should not assume that the accused would reason in the same way as a person without a mental illness. Common assumptions about sane behaviour may be wrong, and the jury should carefully consider any expert evidence (R v Gemmill (2004) 8 VR 242; Taylor v R (1978) 22 ALR 599; R v Matusevich & Thompson  VR 470; Mizzi v R (1960) 105 CLR 659; R v Weise  VR 953).
Expert Evidence About Child Sexual Abuse
A person who has specialised knowledge about child development and child behaviour may give evidence relating to the development and behaviour of children generally, as well as about the development and behaviour of children who have been victims of sexual offences (or offences similar to sexual offences) (Evidence Act 2008 s79(2)).
This evidence may be relevant to a range of matters in a trial, including testimonial capacity, the credibility of a child witness, the beliefs and perceptions held by a child, and the reasonableness of those beliefs and perceptions (Explanatory Memoranda to the Evidence Act 2008).
This evidence may also assist the jury to assess other evidence in the case, or to address misconceived notions about children and their behaviour (Explanatory Memoranda to the Evidence Act 2008).
Such evidence should usually only relate to the general behaviour and development of children who are victims of sexual offences. The evidence is designed to educate the jury and correct erroneous beliefs by showing that “counter-intuitive” behaviour does not detract from the complainant’s credibility. However, it will rarely be appropriate for a witness to give opinion evidence on whether the complainant’s behaviour makes it more likely that the complainant has been the victim of a sexual offence (MA v R (2013) 40 VR 564).
However, the admission of such evidence might invite the jury to improperly reason as follows:
Abuse of children elicits certain behavioural responses;
The complainant exhibited some or all of those behaviours;
Therefore the complainant is likely to be telling the truth about having been abused, or is likely to have been abused, or was abused (Australian Law Reform Commission, Uniform Evidence Law, Report 102 (2005) para 9.157).
It may therefore be appropriate in cases where such evidence is admitted to direct the jury not to reason in this way (Australian Law Reform Commission Report 102, Uniform Evidence Law, para 9.157).
 See "Admissibility of Opinion Evidence" below for more information about the opinion rule.
 For example, where a witness with experience of how a particular type of vehicle behaves in certain conditions gives evidence of that experience, it will not be opinion evidence (Clark v Ryan (1960) 103 CLR 486).
 Whether or not this type of evidence will be opinion evidence will depend on the precise nature of the evidence given. It will not be opinion evidence where it is seen to be evidence of fact about professional practices or professional standards.
 See Identification Evidence for a discussion of the many risks inherent in the process of identification.
 Section 76 states: "Evidence of an opinion is not admissible to prove the existence of a fact about the existence of which the opinion was expressed."
 This includes specialised knowledge of child development and child behaviour, including specialised knowledge of the impact of sexual abuse on children and their development and behaviour during and following the abuse (Evidence Act 2008 s79(2)).
 For example, where the evidence is admitted to establish the consistency of the witness’ conduct, rather than the truth of his or her assertions.
 As this evidence is not being admitted to prove the existence of the facts asserted by the representation, it is not captured by the hearsay rule: Evidence Act 2008 s59(1).
 Evidence that is wholly or substantially based on specialised knowledge that the witness has obtained from training, study or experience (Evidence Act 2008 s79).
 Evidence that is based on what the witness saw, heard or otherwise perceived about a matter or event, which is necessary to obtain an adequate account or understanding of the witness’ perception of the matter or event (Evidence Act 2008 s78).
 For example, opinion evidence may be admitted to establish the consistency of a witness’ conduct, rather than the truth of his or her assertions.
 As the ultimate issue rule has been abolished (Evidence Act 2008 s80), such evidence may now be admissible. See "Abolition of Common Knowledge and Ultimate Issue Rules" above.