Hi Deki,
Did similar for about 5 1/2 weeks, superb time, Did Brisbane to Cairns for about 3 weeks, flew Cairs to Sydney and drove, cross country rather than coast, to Melbourne.
Whit Sundays brill, sailed around them in yacht for 3 days.
Fraiser Island definetly worth doing,
enjoyed the joy of driving and openness,
We didn't spend to much time in any of the cities, sydney like any city really, others a bit unique but the countryside/small towns was where we wanted to be,
Sydney harbour for new years eve was very good, probably only c it once was don't get blotto and miss it!
In terms of advise don't over plan, do as much as you can every day without being under pressure ( Ozzie's don't do pressure!!) Enjoy just being there,
every day will be a new adventure
Getting all nostalgic here .......
Only rule we had was no pub more than once and where possible avoid irish pubs, wanted to go drinking with Irish people could ave stayed at home.