The role and duties of the prime minister are not described by the Australian constitution but rather defined by constitutional convention deriving from the Westminster system and responsible government.

The prime minister is formally appointed by the governor-general, who is ordinarily constrained by convention to choose the parliamentarian able to command the confidence of the House of Representatives (the lower house). Since Federation, this has almost always been the leader of the majority party or coalition. In practice, this means the prime minister most often changes after an election results in a different party gaining control of the lower house or as a result of the majority party changing its leader internally between elections.