The headscratcher for me would lie in the fact that Australia was a stable ecosystem with the various carnivore, herbivore, detrivore niches filled with animals unique to that continent.
If someone were to claim that these animals gradually evolved into those roles over a very, very long period of time, I can see that point of view. On the flip side of the coin if someone were to claim that God deliberately crafted / created those animals to fill those roles, I can see that point of view too.
But would a stable ecosytem be the result of the random introduction of a hodge podge of animals by man? I don't know. It seems like in recorded history, the introduction of animals and plants to the Australian continent from other areas has been a disaster. Australia is plagued with a number of very invasive species.
The European wild rabbit, the American Opuntia cactus ect. not only had no natural predators in Australia, they were not as susceptable to the same diseases as the native flora and fauna.