You've picked the worst possble time of the year.
In my experience its from boxing day on that sites start to fill up
. This year my trip starts 22nd, ends 26th
NO TRAFFIC, AND NO PEOPLE
Now, back on topic... Mungo Brush
is a really nice little spot, maybe for your first stop outta sydney?
But depending on how much driving you want to get over and done with on the first day it might be a bit too close. Its only 3-3 1/2 hrs North of Sydney. You will see dolphins, if not at the beach early in the morning, probably in the bay later on. And, if you get there on boxing day you will see me... rolling up my swag and heading home
As amanda said, try and book if you can. If not, be prepared to be very flexable and have 2-3 parks in mind in each area you want to stop. But I think most of the popular ones have alot of space, there's always room for one more tent. Also, have you got a 4x4? How quickly do you want to get to brissy, do you want to stop 2-3 times overnight each way, or just get there have fun and then drive home sorta thing?