Yes, it is the natural beauty of the place, and a rare example of measured development that has proven to be sustainable (Hayman has been stably developed for decades). Uncrowded as access is strictly controlled, allowing for the ecology to survive and thrive. Helps to have a great climate and scenery. The hotel/buildings/food/service is OK but not the reason to come here (however Hayman is by far the plushest; if you want austere, try Heron Island -- which is interesting in its own right, sort of the anti-Aman).

The other signature resort in Australia is in the desert, by Uluru. I am loathe to recommend it as I don't think it is very good, but some people like it and you should evaluate it for yourself. http://www.longitude131.com.au/