So, unless you believe the 'hordes of empty houses' theory, where are all these people going to live?
Also, what are these asking prices? What has happened in the past is that they'll list their property for sale at a nice price (i.e. what you would have gotten 1 year - 6 months ago) but then when the realise nothing is happening they just take them off the market. Most of these people don't have to sell. They already have plenty of equity, own outright or have jobs.
I think the key factor will be the unemployment figure.
I think you'll sind the answer to that in the OP. The assumption that the number of households per person will keep rising underpins the underlying demand calculations. For example, if you instead assume that the number of households per person will remain flat, then Australia's building slightly more houses than population growth alone requires. And if you take it to the further extreme (i.e. what's happning in other parts of the western world) where the number of people per house rises then we're building too many even at the current lower levels. But understanding of the OP was that the number of households per person is projected to shrink at a slower rate - and that reduces the need for new houses to be built.
That's how underlying demand can show an undersupply - because it assumes the number of people per house must follow the long term trend. Sure, it's the most likely scenario over time, but it's hardly a given, especially in a (hypothetical?) economic enviroment where households are looking to cut costs.
Hope that helps.