The difference between the 2 islands is stark.

In Australia:
Spiders, some of the most venomous snakes, crocs, sting rays, ants that can kill you, etc

In New Zealand:
No snakes, nothing venomous, birds that went extinct because they literally had no idea to run from things

Part of it is because at one point Australia was connected to the Asian continent so all the predators were able to walk there while New Zealand popped up out of the ocean so the only things on it flew there or drifted on wood