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Ok. I've read the story and it's mostly ignorant rubbish. The picture looks like Argonauta argo, though it could be A. nodosa. Both species are known from NZ waters.
As for the whole 'giant tuatua' part, it's bollocks. He most likely found an example of another related (and larger) species. The tuatua (Paphies subtriangulata) is an abundant bivalve from exposed sandy beaches and gets to about 80mm (3 inches) in length. What this ignoramous probably found was a Toheroa (Paphies ventricosa which is morphologically very similar, but bigger, ~120-160mm (~6 inch) bivalve. It could also have been Paphies donacina, but that's not really that important.
What pisses me off here is lay-people pretending to be experts when they essentially know squat...
"It was possible that global warming was making our shellfish bigger, he said."
It's also possible that you are a cheese burger...
Well, it's possible that the reporter also applied a stupidity filter to what the guy said. I am of two minds on this, since I am something of a lay-person pretending to be knowledgable on marine biology, where I know "more than squat but with vast gaping holes." On the other hand, I try to learn enough where I can talk in forums like this, where there are smart people to thwack me upside the head if I say something stupid, and get competent enough that I don't get thwacked too often. I really disrespect people who learn (or make up) just enough so that they can appear really smart to people who don't know anything, but who don't actually care about getting their facts right. And it does seem like the beachcomber may be in that camp (although, in fairness, there are some really smart people who aren't plugged into the academic "peer review" or even "information dissemination" worlds who run open-loop without feedback, and adopt crackpot theories because they just don't interact with people who would be able to point out the obvious flaws.)