Per today’s Melbourne Forecast, issued at 5:06 am EDT on Saturday 10 November 2012.
Forecast for the rest of Saturday Max 20 Partly cloudy. Sunday 11 November Min 8 Max 26 Sunny. Monday 12 November Min 17 Max 23 Shower or two developing. Tuesday 13 November Min 11 Max 20 Partly cloudy. Wednesday 14 November Min 8 Max 22 Partly cloudy. Thursday 15 November Min 10 Max 22 Partly cloudy. Friday 16 November Min 12 Max 22 Shower or two.
As such, I now declare it to be summertime (7 days in a row forecast to be 20+ degrees), and as such am shutting down the gas heater and opening up the cooling vents.
My own use of the gas heater is based on the coldest temperature. Being a weatherboard house (even with the improvements the upgrade insulation has brought), those cool nights (eg the minimums of 8 you’ve cited) are enough to want a bit of heat in the morning to wake up by.