Casual vacancies in the Australian Parliament

dies
resigns mid-term^[1]
is expelled from Parliament and their seat is declared vacant,^[2]
is absent from (fails to attend) the house, without the permission of the house, for two consecutive months of a session,^[3] or
is disqualified.^[4]

^ Constitution sections 19 (Senate) and 37 (House of Representatives). The term "resign" is not to be confused with "retire". A resignation is a voluntary decision by a parliamentarian to end their term early, at a time of their own choosing. A retirement is an involuntary act whereby the sitting member chooses to see out their current term but not to contest the next general election, whenever it is held. The choice of the date of that election is not within the control of the member, but of the government.
^ There has only ever been one such case, that of Hugh Mahon, who was expelled from the House of Representatives in 1920. Under Section 8 of the Parliamentary Privileges Act 1987 (Cth), neither House currently has the power to expel a member. However, since this provision is not in the Constitution but only statutory, the Parliament can amend or repeal it.
^ Constitution sections 20 (Senate) and 38 (House of Representatives).
^ Constitution sections 44 and 45.