Is 'yeah-nah' a uniquely Australian idiom?

Expanding on @jlovegren's comment:

It turns out there are similar idioms in American, Indian, South African and New Zealand English.

This concept has popular culture references in How I Met Your Mother and Punch Drunk Love.

There is a study on this by the University of Pennsylvania.

In the Australian case - there is a study: [K. Burridge and M. Florey, "''Yeah-no He's a Good Kid': A Discourse Analysis of Yeah-no in Australian English", Australian Journal of Linguistics, 22(2): 124-171, 2002. Here's the abstract:

Yeah-no in Australian English is a relatively new marker which serves a number of functions, including discourse cohesion, the pragmatic functions of hedging and face-saving, and assent and dissent.

Also on the Australian side, there is this article in the Age.

So in conclusion, there seem to be similar phrases across dialects of English, but not with necessarily the same meaning. (One might add, their meaning seems sufficiently context-specific and flexible that there are few established rules on this. )

Yeah Nah is not really Australian at all and is far more prevalent in New Zealand. In fact Australian's visiting New Zealand often state they find our use of Yeah Nah baffling.

It's usually used to mean yes but no in a manner such as:

Q) "Do you want to go for a pint after work?" A) "Yeah Nah. I can't today." Translation: I'd love to go for a pint after work but I've got to get straight home today so I can get to parent teacher interviews on time."

Or:

Q) "Is xyz activity dangerous?" A) "Yeah Nah" Translation: XYZ activity is dangerous if you're an idiot.

Yeah nah is a kiwi (New Zealand) slang. We have said it for decades. It is not Australian. Kiwis say yeah... acknowledging what the other person is saying but nah, don't agree or not gonna do that or just meaning no thanks. It's very very common here in NZ. There are even T shirts at our airports with yeah nah written on the front. It's as common as getting up in the morning.