I hear there are also about 4 and a half million homo sapiens there, though presumably they are an introduced species.
There are also significant differences ideologically between the societies as a stereotype.
Politically New Zealand and Australia are different, but I think a lot of that difference is due to Australian federation and our electoral systems. If New Zealand were to join as an Australian state and use the Australian system of preferential voting (instant runoff) I think we would see a convergence in politics. I don't see New Zealand joining the Australian federation anytime soon, but if there was a desire it could happen pretty easily.