Good question, Finn! Before Australia was a nation, it was a collection of British colonies. Each had its own written constitution, parliament and laws. When these colonies joined together to form Australia, a constitutional monarchy system of government was chosen for the new country—very similar to those of the colonies. More information about the Australian Constitution can be found in this written paper and this Constitution video.
Australia, like Britain, is a constitutional monarchy. The Queen is Australia’s head of state and acts in accordance with the Australian Constitution. In Australia, the powers of the Queen have been delegated to her representative, the Governor-General.
As well as being a constitutional monarchy, Australia is also a representative democracy. This means the people vote for delegates to represent them in Parliament. In Australia, members of parliament are elected to the Senate and the House of Representatives to represent the Australian people and make laws on their behalf.