Upvote:7
Nothing will happen (at the airport).
All that said, you should still pay the fine. If you don't pay up, it will start racking up interest and court charges etc, and it may come back to haunt you if they go to collections, you return to Australia for any reason, and you find yourself needing credit, traveling in the NT, etc. You may not have plans now, but who knows what will happen in the future?
Last but not least, if you think $670 is excessive, consider that the maximum penalty for illegal use of fireworks is 40 penalty units @ $149 each, or almost $6,000!