Westpac's Matthew Hassan, alongside the ANZ guys, is probably the best housing economist in the market. Here is his take on the question of seasonality:
"There is some debate around how meaningful the seasonally adjusted price estimates are - for us, there does appear to be a seasonal pattern with a clear wrinkle in prices every Dec (i.e. Dec sales prices are typically 0.5-1% lower) that also makes intuitive sense as this is when buyers tend to wind down for Christmas and remaining sellers tend to be those more urgently requiring a sale (i.e. can't/won't wait until Feb when the market re-opens). Adjustment is therefore appropriate in order to gauge underlying trends."
