Just googled it actually and it’s been written about quite extensively by linguists. It was born In Australia apparently. I even found a little piece on its use by footballers which makes sense:
“The next time a footballer answers “yeah no”, be aware that there is more to the reply than just an “um-ah” prefix. In this sporting context, Professor Burridge says “yeah no” is often used in its abstract context; as a way to defuse a compliment by a bashful footballer.
“You’ve got to downplay the compliment but you can’t reject it because that seems ungracious. It’s a complicated little thing.”