Why do kiwis hate Aussies?

Kiwi are birds. They do not hate anyone. They are just busy surviving in an increasingly shrinking habitat.

As for the non-Maori inhabitant of New Zealand colloquially referred to as 'Kiwis', it is a gross generalisation to suggest they hate Australians. Many of them migrate to Australia. Others who may genuinely dislike Australians no doubt have their own personal reasons, whether it be a bad experience with Australians, or misleading tales gained second-hand from other people with an axe to grind.