And, everywhere, it's about people wanting to live in some specific cities. Even a bit outside of those specific locations isn't necessarily particularly expensive. (Leaving aside ski resorts, especially nice college towns, and the like).
Australia I assume you mean the few places where people actually live in the country vs. the undeveloped outback?
It's just that everyone[1], given a choice, would like to live near the beach in Sydney's eastern suburbs[2], and there is most definitely no more land available in those suburbs. So the only alternative is to build up, and the boomers sitting in their multi-million dollar houses that were originally bought for $3.95 don't like that prospect one bit.
[1] Not everyone, but you get the point.
[2] and Melbourne's inner east, and the desirable parts of Brisbane and Perth.