Hi RoadKing, I agree totally with Mrs Pap. Her Aus travel posts were fabulous and would provide an excellent guide.
I would not recommend driving from Cairns to Sydney it s the best part of 3000 km. Sadly our interstate roads (other than syd -port macquarie syd -melb and bris – sunshine and gold coasts) are from poor to terrible and for large parts single lane undivided roads so european travel time estimates cant apply.
Also ironically the coast roads does nt hug the coast so you just see miles (and miles) of sugar cane or bush unless you detour east.
The exception is the Cairns- Port Douglas trip which is beautiful with Port Douglas being my no 1 recommendation to visit for it s Daintree national park, crocodile infested rivers, beautiful palm tree lined beach and fun tourist/country mix town centre.
I would recommend (time permitting) driving,firstly north from Cairns to Port Douglas and south the further distance to mission beach. Then fly to Brisbane, drive north to the sunshine coast ( including the glass house mountains inland) then back track south to Sydney.
If you want to visit the great barrier reef proper you can fly from Brisbane to central Queensland to visit the Witsunday island area.
If possible check the likely rain predictions in the months you re traveling in North Queensland as the wet season can bet very very wet!
Again all the best and happy travels.