I don't know about "The Folly" site specifically, but the night skies around Ballandean are glorious on a clear night. There should be almost no lights visible from "The Folly" apart from the buildings on-site, and maybe one or two nearby farmhouses. May is generally pretty clear, and it doesn't usually get down to sub-zero in autumn, but you'll still need some winter woolies, as the area is pretty elevated. (Ballandean village is about 720 metres, but you can rise 50 or 100 metres or more as you move away from the valley floor.)
Twinstar Observatory and B&B is located in Ballandean village, and a look at their website will give some idea of what star-gazing can be like on the Granite Belt.
http://www.twinstarguesthouse.com/