Can't speak to Brisbane, as I've not been there, but I'd only go to Adelaide to pick up a rental car to go to Kangaroo Island. I spent 3 days there, and got to explore much of the island...as well as meeting I think the 12 gay residents there.
On the same trip I spent a week on Tasmania, and loved it. I stayed in Hobart, Bicheno, and Launceston, and used those as a base for exploring. Didn't encounter any gay life there, but wasn't really looking for it either. I highly recommend Tassie if you're looking for a quiet getaway.