Rather than which is best for camping, I think the question needs to be which would have Christmas vacancies still. Hop on the phone straight away if they are the dates you want, but I don't really like your chances. Also, a lot of the beachside parks have a minimum booking period over Christmas. Good luck.
It's all fun and games until someone gets hurt - then it's bloody hilarious!
There are several CP's in that area. One a Corindi, then heading South; Big 4, Lorakete, Arrawarra Beach, Mullaway, Woolgoolga, all good parks. Moonee is very nice but watch out for midgies [sand flys] which can be bad at certain times of the year.
Best of those IMHO, is Arrawarra Beach but I like surfing.