This writing prompt is an interesting what-if that would work wonderfully in fantasy or sci-fi, depending on how you decided to handle it.
Unlike most prompts, this one has a very specific location, thought of course you could change it if you want, too. With the various elements listed, it does give a general time frame as well. It has to be after the US and Australia both existed, presumably as nations, and also had some sort of sewer system.
Sewers tend to be a place that people don’t want to go. It’s a dirty place where our waste goes. I can’t imagine these are naturally occurring which begs the question of why were they created. And were they always illegal or was there a time when they were first created that they were legal to use? If so, what changed?
Once you’ve answered all those questions and more, you have some new questions to answer. The first that comes to mind is what are you delivering? After that there are questions of why you and are you from one of those locations or just in them between deliveries?
This is idea is really quite a lot to take in. It has the element of being unbelievable except there is just enough about it that is, unless you have a tendency to spend your time in sewers for one reason or enough, unverifiable, that it makes you wonder. What are your thoughts on this prompt? Would you like to see it in sci-fi or fantasy?