There are far too many surf clam shells washing up on the Whanganui coast, Castlecliff resident Potonga Neilson says.

"As far as I'm concerned, it's a disaster. But nobody gives a damn about it," he said.

The shells are in large numbers and a third of their usual size - perhaps because the population is not recovering from an earlier setback.

The last time a lot were washed up was about five years ago, he said, and conspiracy theories at the time attributed it to some kind of explosion in the atmosphere. But there could be other causes - global warming, different wind directions and changes to surf.

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"The surf has changed a lot. There are different rips, and it could be digging them up and throwing them on the beach."

Last time there were large numbers of shells washed up Mr Neilson contacted authorities about it, and was told it was "a natural phenomenon". He doesn't believe it.

'That was one time. This is two times, so you can't really call it a natural phenomena."

Surf clams are filter feeders that live in the highly oxygenated surf zone. Harvested they make "a lovely feed", he said.

He calls them tuangi. They are from species also known as diamond shell, moon shell and storm clam and are found in water three to 10m deep.

He once inquired about getting quota to fish them commercially.

"They threw a whole lot of bureaucracy at me. They wanted a biomass survey that would have cost thousands."

Marlborough company Cloudy Bay Clams has quota to harvest surf clams from the coast on either side of the Manawatu River. It has considered using Whanganui as a processing base for its North Island operation.