What we did find was rock which had been recently dredged out of the ocean as a harbor and channel widening project in the Islands. This rock was also ancient reef and when cut in half one can see the skeletons of corals, gorgonians, animals and other reef organisms. It is a very porous rock, megascopicaly, and this rock proved to be the prime candidate for use.
there must be somewhere in NZ where a quay has this type of rock structure, I have asked a geoligist mate, so will post the answer if any.....