https://www.lifehacker.com.au/2020/07/difference-between-yea...
https://www.lifehacker.com.au/2020/07/difference-between-yea...
"Ja" is yes. "Nej" is no. "Nja" is neither. It's used to express uncertainty or that it's not as simple as yes or no.
For example if someone asks me if I want to go somewhere with them at a specific time, and I would like to go with them but I already plan to do something else at that time, then I would say “nja” and while I say that think out an alternative suggestion for when we might go.
Another example is if I talk with someone and they say something that I agree with but then say something I disagree with. Then I would say “nja”. In this case it would be similar to English where you might say “yes and no”. And then state what I agree with and what I don’t agree with and why.
It's saying "I hear what you are saying, but I don't agree" with typical Aussie friendly honesty, with the implication that the proposition was unreasonable (often laughable).
Example, "Willing to swap late model Honda Civic for Ferrari or Lamborghini, must be road worthy and registered, cash my way if needed." Yeah, nah.
But a lack of intonation or performance delivery (that head nod or shake) leads to a lot of confusion.
Furthermore, in text form it can be downright ambiguous.
But I do smile when I hear:"Yeah nah, thanks tho' aay."