Waihi Beach locals have been shocked to find thousands and thousands of dead pipi piled up on the beach yesterday.
Eyewitness Jeanette McCallum said she was walking with a friend at the northern end of the beach yesterday when they came across the "shocking" sight.
"We just couldn't believe it," she said. "We were crunching on the shells - they were thick, probably about 20cm thick.
"It was as far as the eye can see ... we were quite shocked and sad."
Ms McCallum, who runs a yoga and reflexology business on the beach, posted a video showing the doomed pipi online, saying she had been heartened by the number of people who had commented on her video in defence of the environment.
She said it was shame, too, as the pipi beds were a welcome food stock in the area.
"We haven't been able to eat pipi for years because of the paralytic virus in them," she said.
"This summer was the first summer we got to eat pipi - and now this happens."
Most of the pipi have now been washed out to sea by the tide, which Ms McCallum said was a good thing, as her husband had reported a terrible smell building up by yesterday evening.
Some have speculated that the death of the shellfish could be connected with extensive flooding in the area experienced last week.
The nearby Top 10 Holiday Park was evacuated after a reservoir uphill began to overflow and fresh water poured down a stream towards the beach.
"I'd say it's fresh water run off from all the rain and that fresh water running down the beach to the pipi beds, that fresh water due to the amount of it probably hasn't had a chance to mix with the saltwater causing the pipi to die," wrote one commenter.
Pipi feed by filtering phytoplankton from sea water and die if left submerged in fresh water for too long.
A Ministry for Primary Industries spokesperson said investigators are looking in to the pipi deaths, but at this stage they do not know what caused the "mass mortality".
"It is not uncommon for shellfish to be affected by big flooding events or issues with water quality," they said.
The spokesperson warned the public against not eating sick or dead shellfish.