Section 51 of the Constitution of Australia

Section 51 of the Constitution of Australia grants legislative powers to the Australian (Commonwealth) Parliament only when subject to the constitution. When the six Australian colonies joined together in Federation in 1901, they became the original States and ceded some of their powers to the new Commonwealth Parliament. There are 39 subsections to section 51, each of which describes a "head of power" under which the Parliament has the power to make laws.

The Commonwealth legislative power is limited to that granted in the Constitution. Powers not included in section 51 are considered "residual powers", and remain the domain of the states, unless there is another grant of constitutional power (e.g. Section 52 and Section 90 prescribe additional powers). Matters covered in section 51 may be legislated on by the states, but the legislation will be ineffective if inconsistent with or in a field 'covered by' Commonwealth legislation (by virtue of s109 inconsistency provision).

The most important heads of section in terms of supporting contemporary Commonwealth legislation are:

Federation was intended to address problems caused by having separate colonies on the one island continent. Section 51 therefore encompasses a group of ‘nationhood’ powers which reflect what powers a ‘nation’ was viewed as possessing. These include:

The Constitution also grants powers over:

Flexibility is allowed by:

The High Court of Australia has the jurisdiction to interpret the constitution, an often controversial ability. Many of the court's interpretations have focused on section 51, from cases arising out of disputes between the states and the Commonwealth Parliament. It was noted in Pape v Commissioner of Taxation [2009] that taxation power under s51(ii) is not "unlimited" and must be employed so as not to discriminate between States or parts of States.

All heads of power in section 51 have an implied incidental area (Lindfield v Marrickville). The incidental area arises when the law operates on persons, Acts, or things outside the actual subject matter of the head of power. Valid laws in the incidental area will have an indirect, but sufficient connection to the head of power. Different judicial tests have arisen to test if the connection is sufficient. The dominant test is if the law in question is a reasonable and appropriate means of furthering an object or purpose in the power (R v Burgess). Other tests are the ‘reasonably necessary’ test or ‘reasonable fulfillment of the purpose’. Justice Mason preferred a 'proportionality' test that took into account the adverse effects of incidental laws.

...
Wikipedia