Maanaki Whenua Landcare Research tested eight dead rats and one weka, but none had any residue of 1080.

Toxicology testing of dead rats washed up on a West Coast shows no presence of 1080, the Department of Conservation (DOC) says.

Locals and DOC staff picked up almost 700 rats as well as dead Westland petrels, weka, octopus, starfish, a goat, crabs, crayfish and skate fish after they washed up on Westport's North Beach on November.

DOC initially presumed the rats died from a recent aerial 1080 drop near the Lewis Pass and washed down rivers with the help of the more than 500 millimetres of rain that had fallen in the region.

SUPPLIED Hundreds of dead rats washed up on a West Coast beach last weekend.

Maanaki Whenua Landcare Research tested eight dead rats and one weka, but none had any residue of 1080.

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Massey University School of Veterinary Science undertook post-mortem examinations of five of the dead rats but could not determine their cause of death. The weka was also examined with cause of death unknown.

Supplied Dead, marine animals, rats and a goat washed up on a Westport beach after heavy rain.

DOC West Coast operations director Mark Davies said the test results confirmed the rats washed up on the beach were unlikely to have come from an area where 1080 had been used.

"We don't know the source of the dead rats but it's possible they came from beech forest areas closer to Westport in the Buller Gorge, affected by flood conditions.

"Rat numbers have exploded in beech forests due to heavy seeding and now seed is germinating, they are desperate for food, which can drive them into new areas and cause them to cross waterways."

Department of Conservation This supplied video outlines DOC's efforts to measure the long term effects of 1080 in New Zealand. They claim in one West Coast valley native bird numbers have doubled. (Video first published in January 2019)

DOC said 1080 would take longer to break down in a rat carcass in the water than one in warmer land conditions, and any poison residue would not wash out of a carcass the way it would wash out of a pellet.

"If the animals had been exposed to 1080, this would have been picked up through the tests."

DOC sent 10 rats for testing. Five went to Massey University for post-mortem, and five to Maanaki Whenua Landcare Research for toxicology testing.

Following post-mortem, the five rats were also sent to test for 1080, although two were too decomposed to do this. The weka was also sent for post-mortem and 1080 testing. The animals were not tested for other toxins.

Davies said it was likely the marine animals that also washed up near Westport were victims of prolonged stormy weather at the time. There had been several recent floods and animals washing up after storms was "not uncommon".