My family in SYD regularly vacations in Noosa, the area is great but the Sheraton there is generally considered fine but unimpressive.

Port Douglas is a great jumping off point for the Great Barrier Reef. That Sheraton also gets mixed reviews, and it's not located all that well for getting to town (rent a car, much better than taking the bus) but it's a fabulous location!

Love the Westin SYD, bathrooms are fantastic and for a hotel in the center of the city is has a great deal of character. Westin MEL is good, too. Haven't stayed at the W (far less convenient as a tourist IMHO) or the Sheraton on the Park. The Four Points is generally a good value, especially as a Plat, but without an upgrade the lower category rooms are quite small.

Starwood is great in Australia, and Australia is great generally as well!