DOC Western South Island Operations Director Mark Davies said it is not inconceivable the rats could have washed out of the hills by heavy rain that followed a Tiakina Nga Manu aerial 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.
While it is possible the rats came from Te Maruia, reports of dead fish and birds, along with the rats, are not consistent with the way 1080 is understood to work.
The poison operation was also more than 140 km away.
"However, until we know otherwise, we have to presume the rats have been poisoned by 1080 and take a cautionary approach," Mr Davies said.
Samples have been taken and urgent testing requested, but any results will take 48-72 hours.
In the meantime, signs have been posted at Westport beaches and planning to work with Regional and District councils to remove the dead carcasses is underway.
