It barely reaches 30 in early October... more like 25-28, which will be cooler past 3pm as 11am-3pm are the hottest parts of the day.
There have been plus 30 degrees days at that time of the year - http://www.bom.gov.au/announcements/media_releases/nsw/20051101.shtml -all I was suggesting was that support for a day-time GF will be lost if it ever coincides with a 30+ afternoon.
The fans would mostly be in their seats at 2pm an hour before a 3pm kick-off (as it is daylight saving time, fans would be in their seats at the equivalent of 1pm).
It may never happen - hopefully not, as high temps don't make for good RL, especially for a GF to decide the premiership - but if it does, then those wanting a twilight or night GF would have all the ammo they need.