What happens if I leave Australia without paying a fine (for setting off fireworks)? [duplicate]

Nothing will happen (at the airport).

1. The penalty for your offense is only a fine, there's no arrest warrant or anything out for you.
2. Fines have a due date by which they must be paid. If you're leaving in 8 days, you will almost certainly leave before the fine becomes due.
3. Australia is a federal state. You've broken Northern Territory law and were fined by Northern Territory police, but Immigration is run by the federal government, and NT fines are not their problem.

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!