Polyukhovich v The Commonwealth [1991] HCA 32; (1991) 172 CLR 501, commonly referred to as the War Crimes Act Case, was a significant case decided in the High Court of Australia regarding the scope of the external affairs power in section 51(xxix) of the Constitution and the judicial power of the Commonwealth.

He further argued that the attempt to make past criminal conduct an offence was an invalid attempt to usurp the judicial power of the Commonwealth, that power being vested by the Constitution in Chapter III courts, by enacting what was effectively a bill of attainder.

[citation needed] By a majority of 6 to 1 (Brennan J dissenting) the court held that the Act was a valid exercise of the external affairs power.

He held that, because the subject of war crimes in World War II was not an external affair at the time, i.e., 1939 to 1945, the subsequent acquisition of citizenship or residence in Australia by an individual was not enough to transform the subject matter into an external affair.

In relation to Polyukhovich's contention that the Act purported to usurp the judicial power of the Chapter III courts, the court held by a majority of 4 to 2 (Brennan J not deciding) that the statute did not invalidly usurp the judicial power of the Commonwealth.