In the Parliament of Australia, a casual vacancy arises when a member of either the Senate or the House of Representatives:
dies
resigns mid-term
is expelled from Parliament and their seat is declared vacant,
is absent from (fails to attend) the house, without the permission of the house, for two consecutive months of a session, or
is disqualified.
Section 44 of the Constitution of Australia
The Commonwealth Electoral Act 1918 (text) requires candidates for Parliament to be Australian citizens.
A member will be disqualified if they are found to have been ineligible for election, or become ineligible to sit, because they:
are a subject or citizen of a foreign power or under an acknowledgment of allegiance, obedience or adherence to a foreign power
are attainted (convicted) of treason
have been convicted and are under sentence or subject to be sentenced for an offence punishable by imprisonment for one year or longer under a Commonwealth or State law
are an undischarged bankrupt or insolvent
hold any office of profit (i.e. income) under the Crown or any pension payable during the pleasure of the Crown out of any Commonwealth revenues, or
have any direct or indirect pecuniary interest in any agreement with the Commonwealth Public Service in any way other than as a member in common with other members of an incorporated company consisting of more than 25 persons.
A member will also be disqualified if they:
take the benefit, whether by assignment, composition, or otherwise, of any law relating to bankrupt or insolvent debtors
directly or indirectly take or agree to take any fee or honorarium for services rendered to the Commonwealth, or for services rendered in the Parliament to any person or State
have been convicted of bribery, undue influence or interference with political liberty, or have been found by the Court of Disputed Returns to have committed or attempted to commit bribery or undue influence when a candidate (the disqualification is for two years from the date of the conviction or finding), or
are of unsound mind.