Out of those two I don't think it's always 100% clear when people mean yes or no. It's usually spoken, not written, so I think it requires context.
"Yeah, nah" would typically be in agreeance of a negated question e.g. "You don't like Game of Thrones either?"
Edit: Heck, even if someone responds simply "Nah" to that question it's not clear what they mean without a further followup sentence or the tone in their voice.
I don't know if it's just Australians or other English speakers, but we definitely have really confusing spoken semantics around affirmation of negated (or even negative outcome) questions.
I have been asked by many a non-native question how to respond to such questions, or even more generally how to use "yes" and "no" in English.
My answer is "use a full sentence". If they insist on using "yes" or "no" because the answer is too long, I say "you can say "I do" or "she isn't", that isn't too long is it?
But if they promise they just want to know how people use "yes" and "no" while communicating and they really really won't try to use it, the answer is it's "yes and no agrees with the main verb in the full sentence of the answer _unless_ the question used a negative verb, in which case the only valid response is 'no'".
If necessary, I can explain: The reason this works is because if you have a question with a negated main verb, the questioner is making an assumption, and you need lots of words to contradict a person's assumption to set them right. Therefore "no" means "your assumptions are validated", whereas "no blah blah blah blah blah" means "let me destroy all your assumptions and rebuild the world in this fashion".
Since I speak with so many non-native speakers, I tend to speak like this automatically. Not every language has words for "yes" and "no".
"Yeah, nah" would typically be in agreeance of a negated question e.g. "You don't like Game of Thrones either?"
Edit: Heck, even if someone responds simply "Nah" to that question it's not clear what they mean without a further followup sentence or the tone in their voice.
I don't know if it's just Australians or other English speakers, but we definitely have really confusing spoken semantics around affirmation of negated (or even negative outcome) questions.