Judiciary Act 1903

A Full Court consisting of less than all the Justices shall not give a decision on a question affecting the constitutional powers of the Commonwealth, unless at least three Justices concur in the decision.

Subject to the last preceding subsection, when the Justices sitting as a Full Court are divided in opinion as to the decision to be given on any question, the question shall be decided according to the decision of the majority, if there is a majority; but if the Court is equally divided in opinion:

in the case where a decision of a Justice of the High Court (whether acting as a Justice of the High Court or in some other capacity), a decision of a Supreme Court of a State or Territory or a Judge of such a Court, a decision of the Federal Court of Australia or a Judge of that Court or a decision of the Federal Circuit and Family Court of Australia (Division 1) or a Judge of that Court is called in question by appeal or otherwise, the decision appealed from shall be affirmed; and

in any other case, the opinion of the Chief Justice, or if he or she is absent the opinion of the Senior Justice present, shall prevail.