Toxicology shows no presence of 1080 in dead rats that washed up along with other animals on North Beach near Westport this month.
About 680 rats, an octopus, a calf and other animals were found washed up on the West Coast beach on Saturday 9 November.
The rats were collected and most were buried following advice from West Coast Regional Council.
The Department of Conservation (DOC) sent 10 rats for tests over concerns they had been killed by a recent 1080 drop 140km away in Lewis Pass National Reserve.
Five went to Massey University for post-mortem, and five to Maanaki Whenua Landcare Research for toxicology testing. Tests returned today showed no 1080 in the rats' bodies.
In a statement today, DOC said Maanaki Whenua Landcare Research tested eight dead rats and one weka, none of which had any residue of 1080 toxin. Two other rats were too decomposed to test.
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.
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 thought it was unlikely that rats had come from our aerial 1080 operation in Lewis Pass National Reserve near Maruia, 140km upstream from Westport.
"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," Mr Davies said.
DOC said it was likely the marine animals that had also washed up near Westport were victims of prolonged stormy weather at the time.