As a rough guide to the southern half of the continent...

Starting in September will give you absolutely huge plants... I'm at around 35'S latitude and usually leave germinating sativas until late October, early November. Most strains will begin to flower somewhere between the end of January and February and finish from late April to mid May.

If you're far north in NSW or southern QLD you will have a larger finishing window (ie you can keep plants in the ground longer) thanks to the milder weather conditions.

A word on frosts... A frost, on its own, wont damage your plants, so long as the days that follow are fine. Problems occur when freezing temperatures are accompanied by lousy weather.