I figure somewhere inland from Carnarvon, around the area they are looking at building the square kilometre array. If they did build, it would have dark sky protection by virtue of the prohibition that would put on development because of the need for radio silence (see
http://www.atnf.csiro.au/SKA/ms32.pdf ). The area also has very good historic numbers for clear sky and low rainfall, at least from the info of the BOM. Just build your observatory to handle the occasional cyclone
I'm not certain how good the access to water would be, though.