Section 2 of the Constitution of Australia says that there shall be a Governor-General to represent the British Monarch in Australia.
The Governor-General of the Commonwealth of Australia is the representative in Australia at federal/national level of the Australian monarch (currently Queen Elizabeth II). The functions and roles of the Governor-General include appointing ambassadors, ministers and judges, giving royal assent to legislation, issuing writs for elections and bestowing honours. The Governor-General is President of the Federal Executive Council and Commander-in-Chief of the Australian Defence Force. All these things are done and all these posts are held under the authority of the Australian Constitution. Further, the Governor-General acts as vice-regal representative to the Australian Capital Territory.
The term of office is at the Queen's pleasure but it is usually 5 years.