Our apex predator was the Thylacine (tasmanian tiger) and it was hunted to extinction by the British that Colonised (stole) Australia. I believe in Natural Selection, the big cats are going to eat all the marsupials which eat all of the insects which keeps our ecosystem intact. They shouln't be here and are only here because of human foolishness. I don't know why domestic cats are growing to Leopard sizes only in Australia though.
Also Australia is barren as ****, nothing can live in the desert, it's much worse than Africa.