In England, they are subjects. The convention's republic makes only cosmetic changes to the Australian constitution. It's probably only a first step, with an American-style presidential system likely in the future. But the proposal offers a sweeping and fundamental change for Australia's civil society. If they accept their republic, sovereignty will be taken from an elderly English woman and distributed among all Australians.
Popular sovereignty is the point of any republic.
Australians will finally be citizens in their own country.