luna-the temps are around 15 degrees at their coldest +- 1
don't drive from Cairns to Sydney, it's a bastard of a drive, and it's 3,000 km, you'll need more than a day or 2!!
I'd suggest mike ball simply because I've been on it and it was fantastic, check their website.
sydney, there are great shore dives and boat diving. Shelley beach, the north head of Freshwater beach (might want to hook up with a dive shop for that one though, it can be a challanging dive in less than perfect conditions), bare island, Kernell, the list goes on...
boat diving... just hook up with a dive shop (I dive with Frogdive Willoughby) and jump on the boat! it only runs on weekends, so you might want to either plan around that or see if others run during the week.
if you want to do land based travel, catch the Cairns Tilt Train from Cairns to the sunshine coast [or stop along the way if you desire], dive the HMAS Brisbane, spend a night in Brisbane, hire a car drive to Byron Bay, dive Julian Rocks, drop the car off at lismore (after seeing wategos beach, Lennox head and other areas around there) fly to sydney and dive.
NO ONE in Australasia, except for those with a vested interest in Cairns, or are holiday divers, reccomends diving off Cairns.. why?? because its shy'te...
Rubbish. don't put your opinion down as fact.