Quality 'beach' campsites are very far in between in the best of situations.
On the Central Coast, there are not all that many I can think of, however have you considered Patonga ??
It's not a caravan park just a council run campground.
It gets VERY busy in peak times, but right now you should get a good chance of getting a spot without neighbours
One one side you have Pittwater and the other a lagoon.
You used to have to call Gosford Council to book a site (very cheap), but I did hear they were looking to put on a full time caretaker, don't know if this happened, so best give Gosford Council a call anyway.
Other than that, we often just head down towards Narabeen 'Big 4' Caravan park, very nice, big lagoon and beach within 5 minute walk.
Hope it helps.....