Citizen scientists making a systematic study of rubbish on a stretch of Castlecliff Beach are finding it's mostly plastic and includes drench capsules used on farms.

A small Sustainable Whanganui group has committed to scouring 100m of beach every three months for three years. It's one in 83 monitoring sites nationwide in the Litter Intelligence project.

It began in April, when Sustainable Coastlines programme co-ordinator Ben Knight taught the procedures.

The group chose 100m of beach near North Mole and picked up inorganic objects 5mm or more in size and within 10m above and below the high tide line.

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On its first visit in April, it found 107 items on a beach that looked clean. They weighed a total of 5kg and 82 per cent were plastics.

The second visit in July picked up 151 items, weighing 3kg and 75 per cent plastic.

Drench capsules were found at both visits. They are plastic and designed to stay in an animal's stomach and leach out substances.

"It's a newer farming item and I'm sure that it's not meant to be washed out to sea at the end of its life," the Whanganui group's data inputter Ron Fisher said.

There was also a lot of unidentified plastic, often in small pieces. Even when too small to see, it stays in the food chain.

The results of those visits, and others nationwide, are up on the Litterintelligence.org website.

The Whanganui group - Fisher, John Milnes, Graham and Lyn Pearson, Margi Keys and Aroha Beckham - can scour the beach in about an hour. Then they head back to the Duncan Pavilion to sort the finds and enter them on the website, which takes another hour.

The citizen science project aims to find out what's on New Zealand beaches, track where it has come from and what currents brought it. When the source is known, the littering can be prevented and Fisher said there have been examples where that has happened.

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Anyone who wants to help with the next beach visit, or any teacher who wants to bring a class, can contact Fisher by emailing ron.enviroschools@gmail.com.