"We have 10% empty homes in Australia (mainly second/holiday...

"We have 10% empty homes in Australia (mainly second/holiday homes)"

Now I don't believe there is any huge shortage of homes, however if a house is not where people want to live then the reality is it is not worth counting as available. There are plenty of vacant houses in country towns where people don't want to live or can't as there is not enough work in the area to support them. If that 10% empty homes statement has any real relevance we need to know where all of these homes truly reside.

"a very low number of people per household compared to 10, 20, 30 years ago and an an increasing number of bedrooms"

True, but on the same token most people will only share if their economic circumstance force them to (unless they have some other over-riding reason to i.e. don't want to live alone).