What proof do we have that Australia exists outside of paid government shills on the Internet?

None. Actually.

Here you're approaching the problem of the unborn birds upon the tree, as described in the Upanishads, and the fundamental question regarding Proof by Inference that troubled philosophers for so long.

Infact if OP were a brain in a cat with a constant simulation running around him, he'd have no way to break free of his containment either, and like the bird, he will eat the sweet and sour fruits of the senses.

There's a solution to this but it doesn't concern us now