You can not get live rock from the beach. You can get rocks with some stuff on it, but it's not liverock. Live rock is coral rock, calcium based and is pretty porous. The rock you'd be getting from NZ because is not coral rock, is not calcium based and is not porous. It may have a bit of coraline and some life on it which could be used to seed dead baserock, but it is not a substitute for live rock.