I know for a fact that it’s mandatory for people who reside in the US (citizens, visa holders or those with a green card) but does that also mean foreign visitors (like those from Mexico, Canada or EU) have to complete the census too when they are on US soil? Have you visited the US during a time a nationwide census was taking place as a foreigner (alien)? If so, do you have to answer the questions present on the census even though you do not live there?
So the government has a rough idea of how many non citizens are in Australia, and roughly where. So any services that non-citizens may need (tourism centres, consulates, etc) get put in the right place.