I can only comment on the NSW South Coast.
Mimosa Rocks National Park. Best spot there (and possibly the only one that you can reach with a camper?) is Gillard's Beach. Secluded beach front camping in the bush and the wildlife is so tame they'd steal the food out of your mouths if they could. Need to take water, toilets and firewood provided. Google it and you'll get all the info from the NSW parks website. There's heaps more on their site too: http://www.environment.nsw.gov.au/parks ... hCoast.htm
They really are all nice and won't be busy that time of the year. No need to book either, so as long as you have a map, you'll be alright.
Mystery Bay primitive camping ground. Not NP, but a very attractive spot. Compost toilets, cold showers (wouldn't get me in them in September!) and taps. You can buy ice and firewood from nearby farms. Just off the Princess Highway.
There's a primitive camping ground in Moruya, but don't know what it's like.
I have never been there but everyone I know says Jervis Bay is stunning. And the area around Kiama is great too apparantly.
Really don't know about North of Sydney, except for One Mile Beach - http://www.melaleucabackpackers.com.au/
My daughter's been there and apparantly it's very nice. They have some sort of a wildlife rescue thing going on there.