Fauna of Australia
The fauna is quite different from that of New Zealand, but is similar to that of New Guinea. That comes about because New Zealand is a volcanic oceanic island with no historical connection with Australia. Its animals have not come from Australia. On the other hand, New Guinea is an island on the same continental plate as Australia, and on the same side of the Wallace line. This was discovered by Alfred Russel Wallace, and explained in a famous series of books. These two huge islands were both part  of the ancient continent of Gondwana, and were not (until modern times) inhabited by animals from Asia. As Gondwana broke up, its two main pieces had marsupial mammals. Eutherian mammals had not arrived from Eurasia.