Kingy
According to this article back in March it was 3.5% in Melbourne with negative equity.
http://www.smh.com.au/business/homes-with-negative-equity-on-the-increase-20120320-1vhe3.html
And likely the proportion has increased since then based on the recent data:
http://www.rpdata.com/research/back_series.html
Even those that took out mortgages with sensible LVR's will be at risk of having negative equity if price falls continue at this pace:
