What other laws the defence power will support has been held by the High Court of Australia to vary based on external circumstances.
In October 1914 the Australian Parliament enacted the War Precautions Act 1914,[5] which gave the Governor-General a broad power to "make regulations for securing the public safety and the defence of the Commonwealth".
(as he then was) observed in Farey v Burvett: I do not hold that the Legislature is at liberty wantonly and with manifest caprice to enter upon the domain ordinarily reserved to the States.
If the measure questioned may conceivably in such circumstances even incidentally aid the effectuation of the power of defence, the Court must hold its hand and leave the rest to the judgment and wisdom and discretion of the Parliament and the Executive it controls—for they alone have the information, the knowledge and the experience and also, by the Constitution, the authority to judge of the situation and lead the nation to the desired end.
In most of the paragraphs of s. 51 the subject of the power is described either by reference to a class of legal, commercial, economic or social transaction or activity (as trade and commerce, banking, marriage), or by specifying some class of public service (as postal installations, lighthouses), or undertaking or operation (as railway construction with the consent of a State), or by naming a recognized category of legislation (as taxation, bankruptcy)•In such cases it is usual, when the validity of legislation is in question, to consider whether the legislation operates upon or affects the subject matter, or in the last else answers the description, and to disregard purpose or object.
In 1949 the Commonwealth used it to support the introduction of the Snowy Mountains Hydroelectricity Scheme, as, although wartime hostilities had ceased, a secure electricity source was needed should Australia be attacked.