Ive never been to Australia, cant speak to the specific situation there, but where I live it is very common for people to have roommates. Many people even share bedrooms to save money. Post covid labor prices for the lowest paid workers has increased substantially. These low paid workers are the most likely to have roommates/shared rooms, and one of the first things they do when they get more money is move into places of their own. Where I live at least you also see the opposite happening. Since housing supply is legally unable to expand people who previously would be living on their own now often find themselves needing roommates, pushing housing demand down.

There are also many people who want second homes and tons of other ways for housing demand to shift without changing population.