I encountered this in Christchurch a couple of times so I can't say for sure if it's a nationwide thing. Instead of saying "How are you?", "How's it going", "How're you doing?", say "How's it?". And don't forget to add a Kiwi accent when trying thus et home. The first time I heard this was in a petrol station. It took me a second or two to decipher.
Petrol station checkout operator: "How's ut?"
Me: "Um... er... oh fine thanks".