Thanks for your question, Vincenzo.
The very short answer is, because it’s in the Constitution!
When the 6 self-governing British colonies decided to federate—unite—in 1901, the new Commonwealth of Australia received a set of rules for how it would be governed. This ‘rule-book’ is the Australian Constitution and the very first chapter covers the Parliament and the Governor-General.
Section 1 of the Constitution states there will be a federal Parliament with law-making power and that it ‘shall consist of the Queen, a Senate and a House of Representatives’ and be known as ‘The Parliament’ or ‘The Parliament of the Commonwealth’.
Section 2 then tells us there will be a Governor-General appointed by the Queen to ‘be Her Majesty’s representative in the Commonwealth’ and exercise all the powers and functions of the Queen.
This means that to make or change a law for Australia, a bill—proposed law—must be agreed to by all 3 parts of the Parliament, including the Governor-General as the Queen’s representative.