Polystyrene is not biodegradable.
Ōwhiro Bay resident Jade Lorier said she was "really worried" about the health of the nearby stream.
"We've got native eels, as well as fish, I'm worried about the wildlife in the marine reserve," Lorier said. "We're trying to protect and restore this area, and this is just an absolute nightmare for the south coast.
"I'd like this person to be held responsible, it's an environmental disaster. I'm furious."
The incident has led to an outcry on local Facebook groups for action against ongoing pollution blown from three nearby landfills on Happy Valley Rd, and from unsecured loads being driven to them.
Lawyer Adam Holloway was among those cleaning up the polystyrene and said there was "constant fresh rubbish" being blown on to the street and the coastline.
"It's disheartening.
"I'm sure we didn't get them all, and next time it rains whatever is left over will flow into the gutters, and from there into the stream, and from the stream into the marine reserve."
A man who asked not to be named told RNZ he had spotted the driver as they were shedding the polystyrene. He followed the trailer into the tip on Happy Valley Rd, where he confronted the driver, telling him he should cover his load, and that the balls he was shedding could kill fish.
He then reported the incident to the council.
"The beans were blowing out like it was Christmas. [The man driving] was a contractor, but he should know better, he could have covered his load.
"Polystyrene is the worst product ever, it's so bad for the environment ... and there's millions of them.
Wellington City Council chief infrastructure officer Tom Williams said the source of the trail of polystyrene balls has been found using CCTV footage, and officers were considering action under the Litter Act.
"The person in question has come round from Lyall Bay all the way round to Ōwhiro Bay, and there's been actually polystyrene all the way round that coast. This is a lovely maritime environment, and a single act of stupidity can ruin it for everyone."
The council spokeswoman said the offender's details were passed onto the Public Health team, which immediately tracked down the culprit, and processed a $200 fine under the provision of the Litter Act.
Council posted about the incident on social media, reminding people to cover their loads to avoid such incidents.
- Additional reporting RNZ