The mystery continues to grow over the hundreds of rat corpses which washed up on a South Island beach this month.
Around 680 dead rats were left strewn across the coastline of Westport's North Beach a few weeks ago - much to the horror of locals.
It was initially believed the rats could have died as a result of eating 1080 poison bait.
However, a recent toxicology report from the Department of Conservation (DoC) found no traces of the 1080 toxin.
"The cause of death is unknown - but the deaths are not a result of 1080 poisoning," said Mark Davies, DoC West Coast operations director.
"We had hoped that the pathology result would provide some more information about how the rats died."
Even the source of the dead rats is unknown.
"The test results confirm that the rats washed up on the beach are unlikely to have come from an area where 1080 had been used," Davies said.
"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."
Other animals to wash up include an octopus, starfish, and a large amount of fish.
DoC believes that it is likely the marine animals which also washed up on the beach near Westport were victims of prolonged stormy weather.
