Camping over N.Y.E - Location Recommendations?

So I've made a last minute(ish) call to go camping over new years with some friends this year. Preferably within a 4 hour drive of Sydney, either coastal or mountainous, large enough to fit around 4 cars and 12-15 people + their tents, camp fires permitted.

Would love all/some/one of either good bushwalking trails / beach / swimming hole if in mountains / room to play some backyard-rules version of <insert sport here> / clear skies for some hobby astronomy. We'd be looking to go for ~5 days, if that makes a difference.

I've been looking fairly hard so far, but most places have either been already booked out (I know, should have planned this earlier), go first come first serve only (which I would like not to risk) or look a bit boring with not much to do (like here ).

I would really appreciate any recommendations for campgrounds that anyone here has had a good experience with over the always-busy NYE period that is likely not to be booked out already. Would prefer to avoid BIG4 style hopiday caravan parks/trailer sites/camp grounds and actually go out to the national parks or even more remote.