Hi dragonfirebcn,
I actually just got back from a trip to Australia, mainly spent in Tasmania (I've been to the other areas on previous trips), and I really enjoyed it. It is very beautiful and lush, similar to the South Island of New Zealand. You could hit the highlights in 6 days. I would say SKIP Wineglass Bay (despite it being on the cover of several Tassie guidebooks)--totally overrated. Yeah, it is beautiful but extremely crowded which detracted enormously from the hike and view. I liked the scenery around the Tasman Peninsula (by Port Arthur) just as much, and it was much more enjoyable. Port Arthur itself is also fascinating. I also liked Cradle Mountain--while it also had quite a few people there, the park is much bigger so it was a much more enjoyable experience than Wineglass Bay.
I would highly recommend adding a trip to Uluru (& Alice Springs, etc.) as part of your trip. It is very unique & special, unlike anywhere else in the world, and definitely worth the detour. I would say that my top three Australia sights would be SYD, Uluru, and snorkeling/diving the Great Barrier Reef.
Personally, I wasn't that excited by MEL. The Great Ocean Road is pretty, but truthfully you don't get the best views from the car, but only if you go out and hike along the headlands (though very easy access from the road).
SYD is great, and you could probably spend all 4 days there. Again, I wasn't that excited by the Blue Mountains tour, and if you have extra time, I'd budget it elsewhere (eg Uluru!).