I agree, don't try to do too much.
I had a 3 week holiday in OZ 9 years ago, and am going again next week for just under 3 (no more holidays

) Flying Etihad to and from Sydney.
First time round:
3 nights in Perth (loved it best),
c. 1 week in Melbourne, and took in the Great Ocean Road (3 nights - saw whales, scenery wonderful) and Philip Island (penguins - but then i have a thing about penguins!)
then we drove to Sydney (took our time, through the national park, and Canberra - 3 nights total),
and spent the balance of the holiday in Sydney (bridge climb etc).
This time it will be Sydney for 1 week - including Hunter Valley for 2 nights, and a night in Blue Mountains, and 1 week in the Whitsundays (flying with Jetstar), to include 3 nights sailing. I will not be diving, but fancy snorkeling, and we're staying in Airlie rather than on an island (on recommendaton of local travel agent).
Have fun planning. And if you want info on Whitsundays, I'm sure I'll be all knowledgeable at the end of Sept!
J