Slang to sympathize with someone whilst disagreeing with them.  Commonly used to politely retort someone.

Example usage:

"Yeah yeah, nah, bro. You gotta pop that shit like this."
- sympathetically feeling someone's frustration at being unable to open a beer bottle and then proposing a solution

And the inverse "Nah nah, yeah" means

NZ Police campaign attempting to sympathize with the common binge drinker

Warning: Phrase is used in Australia, too.