Hey Rob. Not marked on maps as far as i know. We used to line a marker outside Kauri up with a 'sandblow' on fraser. But i think the sand has since been overgrown with trees so its hard to sorta find. Its not a lot you could easily drive straight over it at speed. Current can really race through there and gets quite choppy also at times.
It was really hard to pick up last time i was there (november 2011). But we were lucky as there were 3 boats anchored up very close together in that vicinity
Cheers
Dean