"Seabirds, in particular, will eat these kinds of polystyrene balls, mistaking them for fish eggs," said Ms Easthope, teacher coordinator of Kapiti College's Eco Action Group.
"It's tragic to think our coastal birds and marine mammals might be getting false nourishment from these things and possibly starving to death."
Ms Easthope spent 40 minutes picking up "the heinous things" with her daughter along Whareroa Beach, Raumati South, on Monday.
She believed it was "more than a careless or intentional spill from a beanbag".
"This product is used to keep products cool in shipping.
"At first, I'd thought it was from an offshore source, but now I'm wondering if it's come downstream."
She said polystyrene contained styrene, a suspected carcinogen and neurotoxin, and benzene.
"This stuff shouldn't be manufactured in this day and age and I wish it were banned here, as it is in San Francisco."
A Greater Wellington Regional Council spokesman said a call to its environment hotline on Tuesday described three locations where the balls were visible; Paekakariki Beach by the surf club, Queen Elizabeth Park and Paraparaumu Beach by Manly Street.
"As soon as we were notified we investigated at all three sites.
"We didn't see any polystyrene balls at the first two locations visited, but saw a handful of balls dispersed along the beach at the Manly Street beach location.
"We cleaned up what we could.
"We've also been in touch with Kapiti Coast District Council and are expecting an update today, as we understand more balls have washed up since our initial visit."
He said under the Resource Management Act, enforcement actions for a spill of its kind could range from advice or warning letters through to an infringement notice with a maximum fee of $1,000.
Deputy head girl at Kapiti College Taylor Finderup, who helped to clear up the "absolutely devastating" scene, said she planned to write letters to a number of organisations and councils in relation to the issue.
Miss Finderup, an avid shark fining campaigner and member of the Eco Action Group, said she was extremely disappointed.
"I really wanted to pick up as many as I could, but they were there by the thousands.
"With the marine reserve just off shore, we really can't afford to have these sorts of hazards lying around.
"There've been times large quantities of plastic were intentionally left on beaches, but often it's an honest mistake that some companies end up making, due to faulty transport schemes and packaging problems.
"It was heartbreaking knowing that high tide wasn't far away and they would end up in the sea."
- Kapiti College's Eco Action Group is conducting a Beach Clean Up on Saturday, March 5, coinciding with Sea Week.
Anyone wanting to participate can meet by the boat ramp outside Raumati Beach's Waterfront Bar and Restaurant. Bags and gloves are provided.
