The court must empanel at least 12 jurors in all criminal trials and may empanel up to 15 jurors (Juries Act 2000 s22, s23).
In deciding whether to order additional jurors, the court must consider:
the length of the trial;
the nature of the trial;
any other factor that may result in a juror being discharged during the trial (Juries Act 2000 s23, as introduced by Justice Legislation Amendment (Court Security, Juries and Other Matters) Act 2017 on 2 January 2018).
A judge should only exercise the power to empanel additional jurors when the nature of the trial creates a risk that the number of available jurors will be reduced during the course of the trial. The power should not be used simply as a matter of prudence in short trials, where there is evidence that may be unavailable in a later trial if the jury is discharged. The process of balloting is potentially unfair to the excluded jurors and should not be used unless necessary (DPP (Cth) v Thomas  VSC 61).
A judge does not need to make a formal order for additional jurors. The preferable approach is, however, for a formal pronouncement that additional jurors are empanelled under s23 of the Act (R v Ng (2002) 5 VR 257;  VSCA 108; Wu v R (1999) 199 CLR 99;  HCA 52).
If additional jurors are empanelled, the jury must be reduced to 12 jurors before the jury retires to consider its verdict. This is achieved by conducting a random ballot of juror names or numbers. The jury foreperson is exempt from being removed from the jury by this ballot process (Juries Act 2000 s48).
A fresh ballot must be conducted each time the jury is required to retire to consider its verdict (Juries Act 2000 s48).
In practice, it is generally unnecessary to conduct more than one ballot. Multiple ballots may have previously been necessary when the jury retired to give effect to a directed acquittal or mid-trial plea of guilty to some charges. It is now only likely if the judge splits his or her charge to the jury and directs the jury to return a verdict on some, but not all, charges on the indictment. This procedure is exceptionally unusual and, if there are extra jurors, s48 of the Juries Act 2000 means it is likely that a differently composed jury would consider different charges.
 - This process of exempting the foreperson from the ballot is compatible with the random selection process required in federal matters by s80 of the Constitution (Brownlee v R (2001) 207 CLR 278;  HCA 36; Ng v R (2003) 217 CLR 521;  HCA 20).
 - This procedure is no longer necessary (see CPA 2009 s241).