The Prime Minister is a member of the House of Representatives who leads the parliamentary party, or coalition of parties, with the support of the majority of members in the House. Each political party has its own arrangements for choosing, as well as changing, its leader.
In the case of the current government – the Liberal Party and The Nationals Coalition – the Prime Minister is the leader of the Liberal Party and is chosen, and can be changed, by a vote of their parliamentary party.
The Leader of the Opposition is sometimes called the alternative Prime Minister, as they may become Prime Minister if their party (or coalition) wins the support of the majority of members in the House of Representatives. In the case of the current opposition – the Australian Labor Party (ALP) – the leader is chosen, and can be changed, by a combined vote of their parliamentary party and the ordinary members of the party. The votes of the ALP members of parliament count for 50 per cent in the ballot and the votes of the ordinary party members count towards the other 50 per cent.