Charles is only the king of Australia thanks to the unlawful declaration of terra nullius, and lands were never ceded by most Aboriginal and Torres Straight Islander nations.
So there is a strong legal argument, which was tested and won in Mabo vs Queensland, that the declaration was unlawful.
This technically means the whole colonisation was unlawful and that the Government itself and therefore the head of state have no authority over those native peoples without subsequent treaty.
In a way they’re the only ‘sovereign citizens’ of a Commonwealth state with an actual claim.