The same place that the cash has always come from... when people want somewhere to live, then they will find the cash somehow...
Look at all the cities in the world where house prices are much much higher than Australia... London, Paris, Rome, New York, Seoul, Tokyo, Hong Kong, Singapore, Dublin, Beijing, Moscow etc.
In these cities, people find a way to buy even though the price per square metre has increased to ridiculous levels. Firstly, the units get smaller and smaller in size, while demanding higher and higher rents (which keeps the investors in the market).
Eventually only the super rich can afford to buy or rent close to the city. As a result, the median prices for these cities keeps getting higher, and 'normal' people are forced to live much further and further out in the suburbs. So price growth in the suburbs may not keep up with the price growth in the cities, but it will still grow, because people need somewhere to live. If they are forced to rent forever because they can't afford to buy then so be it... normal people will rent, the investors (and rich people) will buy, and prices will keep rising.
We end up with a two tier system. This is what is starting to happen now in Australia... our property markets are just catching up with the rest of the world. The trend is irreversible, so long as the population keeps growing. The only thing that can reverse the trend is depopulation, which no government will ever consider.
Cheers,
Shadow.