There are heaps of great options! Depends on what type of camping you are into but a good place to start is the National Parks website which lists all camping ground within National Parks in NSW;
My favourite would have to be Caves Beach down on the Bodoree National Park, Jervis Bay, about 3 hrs south of Sydney - flanked by two great beaches, and its relatively small so cant get too crowded. You can drive in and then there's about a 200m walk into the site. Greenpatch is also a good option - you can take your car right in.
If you want to heard west Turon Gates out towards Mudgee is a beautiful remote spot on the river;
Ive also herd some good things about Glenworth Valley - just up on the Hawkesbury, barely an hour form Sydney;
http://www.glenworth.com.au/accomodation-and-camping about 5 years ago