The Department of Conservation says it will not take action against whoever was responsible for culling the birds which are suspected to have ingested poisoned wheat.
Zena Guthrie, of South Canterbury Bird Rescue, said she was contacted by a member of the public who spotted several dead gulls on the beach at Jack's Point, just south of Timaru, on Friday.
She found six dead gulls on Friday and four on Sunday.
The gulls were dying a slow death, she said.
Guthrie said she took one of the gulls to the vet and an autopsy was conducted on the bird, which revealed it had been poisoned.
Geraldine DOC ranger Brad Edwards said they would not be investigating the incident any further.
There was no "by-kill" and after inspecting the site Edwards believed other species of bird were not affected by the poisoning.
"She may want to give them a buzz." However, depending on the type of poison used, Edwards believed the Ministry for Primary Industries could take some action.
MPI did not respond to a request for comment before deadline on Monday.
It was not uncommon for landowners or rubbish dumps to poison seagulls, but people were required by law to do it in a humane way, Edwards said.
Edwards asked people to speak to DOC beforehand and let them know what was happening, and ensure other wildlife would not be affected.
"The culls are done by all sorts of different groups."
Often the people carrying out the poisoning waited until the birds they wanted to target were in the area before putting out the poisoned grain.
The black-backed gull was a native species, but it was not endangered and had managed to thrive, unlike other species of native bird.
Guthrie said the reaction was "just a little bit disappointing".
She was concerned about the knock-on effect it had for other species, such as hawks and dogs, she said.
"It's the effects it's having on other animals. That's all I can really say."
Birds had to be treated as soon as possible otherwise they had "no chance whatsoever".
She encouraged people to let her know at South Canterbury Bird Rescue if they found dying birds on the beach.
"That would be fantastic."
Guthrie said she managed to nurse the bird she had rescued back to health, after treating it with activated charcoal.
She also had a vet do an autopsy on one of the dead seagulls, and was going to get further tests done on the wheat.
"I'm paying for it out of my own pocket," she said.
A member of the Geraldine SPCA declined to comment.
- The Timaru Herald