The Senate is the upper house of the bicameral Parliament of Australia, the lower house being the House of Representatives. The composition and powers of the Senate are established in Chapter I of the Constitution of Australia. There are a total of 76 senators: 12 are elected from each of the six Australian states regardless of population and 2 from each of the two autonomous internal Australian territories (the Australian Capital Territory and the Northern Territory). Senators are popularly elected under the single transferable vote system of proportional representation.

The Australian Senate is vested with significant powers (unlike upper houses in other Westminster-style parliamentary systems) including the capacity to reject all bills, including budget and appropriation bills, initiated by the government in the House of Representatives. This makes it a distinctive hybrid of British Westminster bicameralism and American-style bicameralism. As a result of proportional representation, the chamber features a multitude of parties vying for power. Since 1981, the governing party or parties (which needs to maintain the confidence of the lower house) has only had a majority in the Senate from 2005–2007; otherwise, negotiations with other parties and independents have generally been necessary to pass legislation.