The Boilermakers case, decided by the High Court in 1956, has long been synonymous with the separation of powers in Australia. In Boilermakers a High Court majority applied the separation doctrine to find that the Commonwealth Court of Conciliation and Arbitration could not validly exercise judicial functions. According to their Honours, the Arbitration Court had been established primarily as an arbitral authority and although its judges had life tenure it was not constitutionally possible to give it ‘any part of the strictly judicial power of the Commonwealth’. This included the power to interpret and enforce industrial awards. For the Boilermakers' Society of Australia – the applicant in the High Court proceedings – this was a significant victory for it meant that the Arbitration Court could not penalise the union for its involvement in strike action in breach of award.

Despite its prominence, the legal significance of Boilermakers is not always understood. It was not the first case to recognise that the Australian Constitution incorporates a separation of federal judicial power from legislative and executive power. Before 1920 it was already apparent that the judicial power of the Commonwealth could only be exercised by the courts listed in s. 71 of the Constitution: the High Court, federal courts created by Parliament, and courts invested with federal jurisdiction (‘Chapter III courts’). This is commonly described as the ‘first limb’ of the separation of federal judicial power.