In an effort to clear a courts backlog, minor infractions are handed over to AI to adjudicate. However, because it has been trained on public domain material, the AI keeps handing out antiquated judgements like 'transportation to the colonies'. Of course, Australia doesn't want them, so sentenced individuals are forced to behave as if they were in Australia: only go about at night, observe a mirror calendar, be treated by everyone else as if they were thousands of miles away. To be 'Down Under' is to be a sort of living ghost.