Hi Steven,
 
The "Cymbio" prefix of the name suggests either Cymbiolacca or Cymbiolista,
both genera in Volutidae.  "numtini" sure doesn't ring any bells.  Just
looking at the name you supplied, and thinking of all the illegible hand-
written labels I've dealt with, the name that pops into my mind is Cymbiolista
hunteri.  The locality presents a problem though - the only South Island I
know of is in New Zealand, and none of the Cymbiolacca or Cymbiolista volutes
come from there - I believe they are all Australian except one from New
Caledonia.  Of course, there could be a South Island somewhere else??
A brief description of the shell would help!
 
Paul M.