The Commonwealth of Australia was established on January 1, 1901, and on that day the Australian Constitution came into effect.
The Constitution establishes a framework for the governing institutions; prescribing the powers of the Federal Parliament to make laws and created a Federal Judiciary to uphold them.
The federal parliament representatives are elected by the people, the majority party forms the Government and they choose their leader who then becomes the Prime Minister of Australia. Government representatives are all public servants. They are meant to represent their constituents first, not their party agenda, this has become skewed through time.
The role of the Cabinet government, the parties, and the Prime Minister is nowhere mentioned in the Constitution; they are customary offices, grandfathered in and accepted!
ANYONE can form a political party who meets the criteria. Labor and Liberal have been the predominant parties through Australian political history, alternate parties are known as independents. The two party system made “forming government” easier, but it’s not constitutionally grounded. If more independent representatives are elected by the people, then coalitions are formed to create a majority to form a Government.