Why is there a British flag on the top left of the Australian flag?

Australia is a Commonwealth country and was once a British colony, but not all Commonwealth countries have British flags, such as India, because Australia's head of state is the Queen of England and the British Governor (but the Australian Prime Minister has actual power, so Australia is an independent country), and India's head of state is elected by himself. However, there are some exceptions. For example, the head of state of Canada is also the Queen of England, and it is an autonomous territory, but Canada was replaced with the Maple Leaf Flag with the approval of the Queen of England.