Hundreds of dead rats have washed up on South Island beaches, prompting a warning from the Department of Conservation (DoC).
DoC issued a warning on Saturday, telling dog owners to keep their dogs off Westport beaches after hundreds of dead rats washed up there over the last 24 hours.
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Western South Island operations director Mark Davies says it's "not inconceivable the rats could have washed out of the hills by heavy rain" that followed a 1080 operation at Te Maruia in the Lewis Pass National reserve recently.
"The Te Maruia and Inangahua river catchment feeds into the Buller River and had a plague of rats treated with toxin a week ago," the statement said.
But while it is possible rats came from there, "reports of dead fish and birds, along with the rats, are not consistent with the way 1080 is understood to work".
That was also 140km away.
However, Davies said until more was known, it would be presumed the rats were poisoned by 1080.
Samples have been taken for urgent testing, which may take between 48 and 72 hours.
Signs have been erected at the affected beaches and work is being planned to remove the rats.