Let's see what data shows:
|Australia Property Investor/FHB market share vs. house prices|
According to FED property investor market share in bubble states increased from less than 25% in 2000 to almost 45% in 2006. Similar rise is property investors was recorded in Australia as a whole. In some areas (SE Qld, inner capital cities, ...) significantly higher percentage of investors entered property market and drove house prices by increasing debt to extreme levels. It is hard to imagine this trend to continue for long because of huge debt already accumulated. Once prices stop growing, property investors begin to sell because quick capital gain expectations are gone while big repayment bills continue to arrive every month. Most of highly leveraged and negatively geared investors cannot afford to hold for long without quick capital gain expectations.
Looks like same forces created housing bubbles in USA and Australia. Other minor factors made bubble extents and timings slightly different but core cause for rise and fall seem to be the same.