Washed up on a tidal bank. I see on the taxon page "Still working on classifying this taxon. Check back later".

I got this name from http://www.mollusca.co.nz/speciesdetail.php?speciesid=2303&species=Limatula%20%28Stabilima%29%20maoria and considering it has been found north south and in the harbour from here, it has to be this!