But that has nothing to do with "yeah nah" and "nah yeah" and everything to do with "yes" and "no". If someone makes a false assumption, which is what that question necessarily entails, you must use many words to set them right. If someone has not made a false assumption, as "Are you going on Wednesday?" you can give them the answer with a single word.

Likewise if they said "You're going on Wednesday right" you could say "yeah" but you can't say "No", you must say "no, I'm going on Tuesday" or "no, it's my kid's birthday" or "no Clive is".

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).