You could always try one of the Big4 parks near sydney or even down the south coast.
We stayed in a Big 4 in Swan Hill, VIC Xmas hols (Boxing Day onwards) last year and are staying at the one in Deniliquin this Xmas with my parents (caravanners) and my brother (cabin as his wife doesnt camp!!). We go more for the river spots rather than coastal as they are usually chokkas and hubby loves to fish and we also waterski.
There's a Big 4 at Lake Conjola. Not sure what the parks like but it's a beautiful spot as the in-laws have a house near there at Manyana, only problem would be that the only shops are the general store. Very pretty part of the world though.
Also there are supposed to be some lovely national parks in the Blue Mountains, a girlfriend was talking about one the other day near Glenbrook. But I don't know much about it but someone else on here may.
Good luck with your preparations and who knows maybe next year they will go a bit further afield????