How far are you prepared to drive??
You could go out around Moss Vale, Kangaroo Valley, Bargo or anywhere in that Sth Highlands region. You'll still get some light pollution (Sydney is a pretty big, bright place), but you'll get out of most of it. Expect a two hour drive or so to get out of the worst of it. You could make a weekend of it and camp out with your gear
It's been awhile since I was down that way (a few years), so hopefully someone from down there will chime in. But any place in that area to the SW of Sydney will be fairly good. Try to stay out of the bigger towns like Bowral and such, although they're a lot better than being closer into town.