Burraga Dam (-33.935756,149.554061) is a very dark sky site. Amenities might be a little spartan though. There is a toilet, but not a shower as far as I know (it's been a couple of years since I've been there). You can swim in the dam though.
Abercrombie Caves would be another possibility for camping and good amenities but from memory it's very deep in the valley so the view of the sky is limited to the highest parts only. Not sure if Wombeyan Caves are any better
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Boyd River Camping ground in Kanangra Walls NP has toilets only, but the creek runs through the campground - might be difficult to find a good clear view of the sky due to trees though.
Flat Rock camping area is too close to the road for astro IMO.
Al.