Excellent thesis from 2012 on Cool Climate Wines, http://www.wsetglobal.com/documents/wineaustralia2012.pdf

How is cool climate defined in Australia?

As in other wine producing countries there is no official definition of cool climate in Australia.

One possibility is to count the heat
degree days HDD (average
temperature per day (°C) minus 10
multiplied by the number of days in the
seven month growing season). The
national cool climate wine show in
Bathurst defines a cool climate region with heat degree days of 1600 or less.