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.
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.