"We have approached a situation in Australia where some property can be replaced / rebuilt cheaper that its sale price,"
If you are talking of the house it's self, that is quite common.
The are many areas that have older houses on them and basically you are only paying for the land and if it has been rezoned and sub-dividable it would be worth more if you demolished the house.
As for FB post, he could also use the example of Detroit where people have walked away from home. The thing to remember is that in these places you have a mass exodus of the population, in the case of Detroit because the auto industry closed down and there was no jobs.
As I have said before the only way we would have a major crash of the property is if we had a major exodus of the population and that's looks highly unlikely as we have forecasts of a growing population
