A group of activists have sent out a letter saying anyone involved in an upcoming 1080 drop is "committing an act of terrorism".
Police on the West Coast are investigating the origin of the letter, sent to "a number of organisations" in the area.
"The 1080 programme is lawful and the belief that anybody associated with this is committing an act of terrorism is absolutely incorrect," said Insp Mel Aitken.
"While all people have the right to freedom of speech as well as peaceful protest, I wish to make it clear that threats made towards individuals or organisations that are involved in the 1080 programme will be treated seriously."
A five-year study published last year showed 1080 poison was behind a native bird boom, and it's also been credited with wiping populations of rats and other pests.
Anti-1080 protesters have ramped up their opposition in recent years, including loosening wheel nuts on cars belonging to Department of Conservation workers and threatening to "take down" helicopters.
- 1080 protesters threaten to 'bring down' DoC helicopters
- Forest and Bird condemns anti-1080 attacks on DoC workers
A man who threatened to poison baby formula with 1080 was jailed in 2016.