Juries Act 1981, ss 23–26

The names of all the people who have been summoned for jury service on that day are placed in a ballot box. The court registrar then draws names randomly from the box to select the people who will serve on the jury. If your name is called, you go forward to take a seat on the jury.

Lawyers for the prosecution and for the defence each get the opportunity to “challenge” those selected before they sit down. If you’re challenged, you can’t serve on the jury. Each side can challenge up to four possible jurors without having to give a reason. In addition, they can challenge jurors on the basis that they won’t be impartial in the case or that they have some disability that prevents them acting as a juror effectively.