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If you were Prime Minister you could create your own party, but you might not want to …
The Prime Minister is leader of the team, or coalition of teams, with the support of the majority of members in the House of Representatives. They are elected by the members of the government, and can keep their job as long as they are a member of parliament and retain the support of the government.
If the Prime Minister chose to leave their team and create a new party, they would lose their position unless their new team now had the support of the majority of members of the House of Representatives.
You can discover more about the role of political parties here and specific details about a particular political party by visiting their website.