In keeping with the general spirit of the Herengracht index and Dutch canal-side housing, it is my opinion that overall the most rational proxy for median residential property price growth over time is household income growth.
That said, I do think that we need to make some allowance for the structural shift towards lower interest rates in recent times, which has naturally led to an increase in household debt levels. It's been often argued that debt levels are unsustainable, yet they have been sustained for many years now.
Take a read of this Business Spectator article where Christopher Joye introduces some interesting arguments - and a couple of interesting charts:
Of course the figures won't be accurate for there are simply far, far too many variables for anyone to know.
It is worth noting the difference here, though, between real and nominal prices.
Of course we can't know accurately what this means for property prices in absolute terms. It depends to a large extent upon whether we deflate by inflation or by household incomes (which are likely to be higher than CPI), but it is worth noting that over that timescale even Australia's most ardent doomsayer is predicting no kind of material absolute price falls at all.
If you are in property for anything less than Keen's 15 year time horizon, you are to some extent taking a punt on a whole host of unknown outcomes (which is fair enough, but should nevertheless be acknowledged).