Mystery plastic pieces are baffling beach cleaners in Wellington. After beach clean-ups, plastic 'nurdles' just continue to resurface.

It took 15 volunteers just five hours to collect 200 kilograms of plastic nurdles in a cleanup of Evans Bay.

Using "wheelie bin" filters, Petone Beach Clean Up Crew co-ordinator Lorraine Shaab said the amount collected was equivalent to the weight of 40,000 plastic bags.

Nurdles are the raw material used to make anything plastic, and they're about the size of a lentil.

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They don't disintegrate or break down into smaller pieces and can be harmful to animals which mistake them for food.

Shaab said they would be back at Evans Bay this weekend, and expected to collect even more than 200kg.

The wheelie bin filter, designed by Dave Pine from Synapo, allowed them to sift through the sand faster.

Volunteers shovel sand in to the bin, the nurdles float to the surface, and the sand sinks to the bottom. The water rises and the nurdles flow with excess water in to a filter bag.

Weighed and measured, they finished with 10 bags, each with about 20kg of nurdles.

SUPPLIED Beach cleanup volunteers collected about 200kgs of plastic waste from one part of Evans Bay in just five hours.

Shaab said she returned to the area where they cleaned just weeks afterwards, but the nurdles had returned. "The problem is not being fixed."

In June, plastics manufacturer IML Plastics was accused of spilling thousands of them into Wellington Harbour – but the company says they've put filters in place, and only collect a small amount each time.

Manager Richard Jorgensen said the plastic could not be coming from their business, according to the flow of the harbour and the tests they had conducted. ​

"It could have also come from a number of other sites in the Wellington area which have closed down."

SUPPLIED The wheelie bin filter system used by beach cleanup volunteers was designed by Dave Pine from Synapo.

The plastic could already be decades old, he said.

Jorgensen had two theories. One was that the nurdles were arriving in the ballast water of ships arriving in Wellington Harbour from overseas.

The second was that they were coming from a cleanfill used by the construction of Wellington Airport's runway

But a Wellington Airport spokeswoman said the airport didn't use plastic in the cleanfill in the runway when it was constructed in the 1950s.

"The cleanfill used is comprised of rocks and dirt sourced from the nearby hills."

A Ministry for the Environment spokeswoman said the Ministry was not investigating the source of the nurdles, as it was a local issue.

A Greater Wellington Regional Council spokesman said there were no regulations relevant to them which would enable them to control the use of nurdles."