170mm long.
1.5 - 2mm deep outer shell eaten away by grazer or sponge chemical attack?

Holes and burrows made by sponges, boring bivalve molluscs [st: I don't know if NZ has any of these; gastropods yes], polychaete tube worms:
http://natureinfocus.wordpress.com/tag/epibionts/
"In common with the borings of the Polydora worms, it is thought that the acidic metabolic waste products of the organism gradually dissolve the shell to create spaces within the shell matrix to accommodate further growth of the sponge."
Bottom, Polydora ciliata type burrows look similar to this mussel.

http://www.fish.govt.nz/NR/rdonlyres/24D38008-1A94-41EB-BB05-E1E94F050F0F/0/2013chapter7biosecurity.pdf
"Green-lipped mussels in New Zealand have been infested by both Polydora and Boccardia species,"