I agree that Adelaide really wouldn't be on my list if I only had a short time, there really is nothing to do there. Don't get drawn in to Cairns either if you up north. There is a gay resort near Port Douglas called Turtle Cove, I haven't been there but my partner and I are going to be visiting northern queensland in March 09 and were thinking about it so any suggestions also greatly appreciated.
Bedarra is lovely (would avoid Dunk unless you want a holiday camp style holiday full of kids) although it is very remote so if you want night life that isn't the right place. Port Douglas also gets a big thumbs up from me, Sheraton Mirage used to be a great spot but haven't been back for a while.
Sydney is 100% definite. We stayed at the Sheraton on the Park for 10 days over Mardi Gras last year and it was a great spot. Had previously stayed at the Intercon which is also pretty well located. If you get a chance I would recommedn Iceberg's at Bondi for dinner one evening before heading out to Oxford St for the bars. You will love Australia regardless! Wonderful place with wonderful people!