A scientist believes last week's torrential rain is behind the shock discovery of millions of pipi washed up on Waihi Beach.
Locals walking on the beach were saddened to find the shellfish on the north end of the beach on Tuesday.
The Bay of Plenty Regional Council said in a statement this afternoon the rain last week was the likely caused the pipi to go to higher ground.
Staff from the council and University of Waikato collected samples of the shellfish yesterday afternoon and will be analysing these for the presence of toxins.
Samples have also been sent to the Ministry for Primary Industries to rule out any disease cause.
The Regional Council funds a Chair in Coastal Science role to provide additional science support for issues like this.
Professor Chris Battershill, the current Chair in Coastal Science, said it is likely a number of factors contributed to this shellfish die-off.
"Apart from the weather bomb that hit the area last week, higher ocean temperatures are being recorded in this area and there have been some large sea swells," he said.
"It is also possible that this is a natural die-off due to stress from overcrowding as there is a very large population of shellfish densely packed just off the beach in this area," he added.
"Although most of the shellfish appeared to be dead we did observe many live shellfish within the wave zone," Professor Battershill said.
The council says it expects to have some results back within three to four weeks.
Jeanette McCallum said she was walking with a friend at the northern end of the beach on Tuesday when they came across the "shocking" sight of the pipi.
"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."
Most of the pipi have now been washed out to sea by the tide.