“They can’t, they’re Australian citizens.”
Not can’t, won’t. Nations have been revoking citizenships since it was invented and they can easily do so here. The problem is out-of-date laws and a lack of political will to change them (i.e. political correctness).
Without checking, I am fairly certain that the Australian constitution bars them from revoking the citizenship of nationals born in Australia, just like our constitution does.
I like your tag line BTW.