If members of parliament are elected through a party, how can they leave that party and still keep their seat?

According to the rules in the Australian Constitution, once someone is elected or appointed to the Australian Parliament, they hold their seat as an individual. There is no law that requires them to stay in the same political party. Even if they were chosen by their party to fill a casual vacancy in the Senate, they are not legally required to remain with that party after being appointed to the Senate.

At a federal election, members of parliament are voted in by the Australian public as individuals. They may be a member of a political party, but the voter is indicating which person (not which party) they are voting for. Voting works differently in the Senate and House of Representatives:

• In House of Representatives elections, voters mark their choice next to a person’s name on the ballot paper. The ballot paper also shows the candidate’s political party, if they belong to one, using a label and logo.
• In Senate elections, over 90% of Australians vote ‘above the line.’ Votes cast above the line are marked next to a party name and logo. This may give the impression that the vote is given to that political party, however, these votes are still for individual candidates. By voting above the line, a voter is marking their preference for the candidates in the order that party has listed them below the line. The vote is still counted for individual people. Voting ‘above the line’ is a bit like using a shortcut to direct votes to the individual people determined by that party, as shown on the ballot paper below the line.

This means that, regardless of how a person has been elected, each member of parliament is elected as an individual. If they choose to leave their political party, they are allowed to stay on as a senator or a member of the House of Representatives.