Brit here who has lived in NZ for a few years. Polled some Kiwis in the vicinity with your example and "yeah nah" is very much negative, even in that instance.

Brit here who has lived in NZ for a few years. Polled some Kiwis in the vicinity with your example and "yeah nah" is very much negative, even in that instance. I agree that on paper it's a double negative but I suppose we have to be descriptivist! As far as I can tell, a sibling comment has it right: the last word signals the intent (modulo tone, sarcasm, etc).