Negative gearing (a way to merge unlimited losses from investment property into your personal tax return) and the core, multi-generational opinion that investment property is the best way to build wealth in Australia and no property tax (equivalent is rates which are significantly lower on average than the US and stamp duty which is only payable on the sale of a property) has causes massive investment in non-PPOR properties as well as massive property inflation costs in general.

Oddly enough, a very similar percentage of homes are owner occupied between the two countries despite the apparently dramatic differences in mortgage availability.

Edit: this is based off a two minute Google I could be wrong.