Righto, I got off my arse and rang NPWS, or whatever they're called this week. They confirmed that they will not take any bookings, but that they had not had to turn anyone away that they could recall.
There is a caretaker at Diamond Head, but they're not publicly contactable.
I I found this website, although the GPS reference given here and that I get from Garmin Mapsource vary slightly. Punch it into Google Earth for a gander.
Kylie's Beach Campground . Everyone clear on how to get there?
Importantly, there is no drinking water at the site, so please be sure you pack a jerrycan or two. I believe clean water can be obtained from Diamond Head, so we should not run short in any case.