Clamped

Hey there.
I have been the victim of car clamping, and I am pissed off about it. I was trying to do the respioncible thing and avoid congesting the road with my car by taking the train.
The first car park was full, so I followed the large, N.Z legal roadsign to the next carpark, locked my car and left it. It turns out it was private, however their were no signs on the entrance, and no sign in sight where I parked. (they were hidden behind parked cars)

Is this clamping legal?
Cheers,
Mungus