Australia will keep the monarchy until at least the Queen dies. She is still liked there by many. It will be a voter turnout problem for the republicans. There is also some benefits of being in the Commonwealth that the Aussies won’t want to readily give up.
Voter turnout will not be a problem. It’s mandatory universal suffrage.
Even if the Queen is dumped that need not affect the Aussies’ Commonwealth status. Most commonwealth countries do not have her as Head of state. She has the other role “Head of the Commonwealth” for her symbolic link to the group though..