There is a NP campsite at Trial Bay Gaol called Arakoon, where we stayed a year or two back. It is an expensive place as you have to pay a NP entry fee, then a campsite fee and then coins to use the showers. (Apparently it stops surfers washing their boards in the hot shower water!!)
We did a rush set up because we arrived late in the afternoon and wanted to do a tour of the gaol, which is money well spent. It's a fairly popular place so a call to the NPWS office to check if there are campsites available would be advisable I think. Only stayed there overnight on our way south from north, visiting the parents @ Maclean.