Polyukhovich v Commonwealth

32. The absence of any similar prohibition in our Constitution against bills of attainder and ex post facto laws is fatal to the plaintiff's argument except in so far as the separation of powers effected by our Constitution, in particular the vesting of judicial power in Ch III courts, imports a restraint on Parliament's power to enact such laws. In this respect the prohibition against bills of attainder has been seen 'as an implementation of the separation of powers, a general safeguard against legislative exercise of the judicial function, or more simply - trial by legislature': United States v. Brown (1965) 381 US 437, at p 442. This doctrine applies to bills of attainder but not to the generality of other ex post facto laws. That is because it is of the essence of the prohibition of a bill of attainder 'that it proscribes legislative punishment of specified persons - not of whichever persons might be judicially determined to fit within properly general proscriptions duly enacted in advance': Tribe, American Constitutional Law, 2nd ed. (1988), p 643. The application of the doctrine depends upon the legislature adjudging the guilt of a specific individual or specific individuals or imposing punishment upon them. If, for some reason, an ex post facto law did not amount to a bill of attainder, yet adjudged persons guilty of a crime or imposed punishment upon them, it could amount to trial by legislature and a usurpation of judicial power. But if the law, though retrospective in operation, leaves it to the courts to determine whether the person charged has engaged in the conduct complained of and whether that conduct is an infringement of the rule prescribed, there is no interference with the exercise of judicial power.

A majority of the High Court shared Chief Justice Mason's conclusion in the validity of the War Crimes Amendment Act 1988. An amendment to that act could easily encompass war crimes committed in Afghanistan during the rule of the Taliban.