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It's just filler really. I don't think I've ever said "yeah nah" without adding on something else afterwards to clarify which one I mean.

Agreed it is just filler. It's closer to "But" or "Um" or "You know" than a "Yeah" or "Nah".

This song will clarify it from an Aussie perspective: https://www.youtube.com/watch?v=nyadntqXjn4

It does not clarify it at all. It merely uses it. If you don't know what it means, you still don't.