What happens if a member of parliament leaves? Do they get replaced?

Yes, they do get replaced! How they are replaced depends on whether they were a member of the House of Representatives or a senator.

If a member of the House of Representatives resigns or dies between federal elections, they will be replaced in a by-election. A by-election is an election held to fill a single vacancy in the House of Representatives outside of the regular 3-year election cycle.

If a senator resigns or dies while in office, this creates something called a casual vacancy in the Senate. The rules for filling casual vacancies are in section 15 of the Australian Constitution:

• The parliament of the state or territory the senator came from appoints – chooses – a replacement to fill the casual vacancy.
• The newly appointed senator must be a member of the same political party as the senator who has left.
• If the senator who left was an independent or was elected as a member of a party that no longer exists, the state or territory parliament with responsibility to fill the vacancy decides on an appropriate replacement.
• If the senator was elected as a member of a political party but left that party to become an independent or join a different party, the casual vacancy is filled by someone from the original party.
• The new senator serves the rest of the original senator’s term in the Senate.