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A wildlife veterinarian says he strongly suspects that hundreds of gulls found dead on beaches north of Wellington were poisoned.
About 150 dead black-backed gulls have been found on beaches stretching from the Kapiti Coast through to Manawatu since Friday.
Beverley Dowling says the scene on the beach north of Otaki on Friday was absolute carnage, with dead and dying birds blanketing the shore.
"I have never seen such devastation, I've never seen anything like it," she said.
"Everywhere you went there was dead birds."
Claire Thornton from animal welfare group HUHA in Otaki said she and a team rushed to the scene and saw birds tumbling from the sky.
"They were gasping for breath, they were suffering and it was horrible to see.
Brett Gartrell from Massey University's wildlife hospital said it was highly likely the birds were poisoned but it was too soon to tell if it was been done deliberately.
He said of the 20 birds brought to the hospital on the weekend all but two had died.
"These are adult birds in good body condition dying very suddenly.
"We know it's not a starvation event, we know that it's not a disease-event based on the signs that we've seen so far.
"So we are thinking about a toxin of some kind but we can't be sure whether this is a natural toxin that's occurred in the environment, or whether this in fact has been caused by some humans."
Prof Gartrell said post mortems of the birds have begun but it could take weeks for tests to confirm what killed them.
But he said based on their symptoms before they died the most likely explanation was poison by a commonly used over-the-counter pest-control substance, alphachloralose.
"[It] is a sedative anaesthetic type of poison and that would explain the very low and erratic heart rates and the muscle weakness and collapse that we saw."
The gulls are one of the few native bird species that are not protected, and can be culled by private landowners without any requirement to notify the Department of Conservation.
DOC and the local and regional council all said they were not doing any culling or pest control in the region that could have affected the birds, and were not aware of anyone else doing so either.
Dowling said she had heard from multiple people of vehicles driving on the beach in the dead of night in the past couple of days.
She said it was possible a person was buying bags of fish bait, lacing it with poison and leaving the fish strewn around the beach for the gulls to eat.
Dowling said a person she knows had a scary encounter with someone on the beach this morning.
"The person who found the bait bags this morning .... a car came over ... a man charged out of the car and literally snatched one of the bait bags off them [saying] that 'belonged to him' and was very threatening.
"So there's clearly something going on, but we don't know what, or who."
DOC said it was working with other agencies, including the regional and district council and Ministry for Primary Industries, as it tried to figure out what was happening.