There isn’t really anything here that would prey on a cat. Australia is pretty harmless comparatively. No bears, no ‘big’ cats or wolves of any kind. Crocodiles are limited to the tropics and most of Australia is not in the tropics. Nothing like anacondas and cats are too quick for snakes anyway. There are sharks but they pose no threat to cats. For all the press Australia gets about dangerous wildlife it’s actually a pretty chill place. The cities are pretty devoid of bugs if you live in a flat or keep your garden litter down, even more devoid of bugs since I was a kid (mostly due to massive logging in regional areas and overdevelopment in urban areas).

I would think cars would probably be cats biggest killer, especially in urban areas.