These were common along the rocky shore, but not nearly as abundant as the half crabs (Petrolisthes elongatus), which were everywhere in huge numbers. Those were the two crab species I found.
Probably Pilumnopeus serratifrons. Under stone, intertidal zone.
Near the rock pools, South Piha
From what we could see of this crab, it had red legs with a mottled black gray and slightly green carapace