An anti-1080 activist says he was shocked when about 10 police officers turned up to search his home near Nelson.
Rolf Kleine said he was questioned this week and had his home searched. He denied any involvement in the blackmail threat and said he had no idea who was behind it.
Mr Kleine said he could understand why police were interested in speaking with him. Three years ago, he had sent a polite letter to Fonterra about his concerns with 1080 and the possibility that 1080 residue could end up in milk products, he said.
During questioning, he said police were most interested in his online activity.
"Most of those questions, I don't know, maybe understandable, were regarding letters to editors. Let's say I wrote letters to editors at Nelson Mail, to the Dominion Post.
"That was in 2011, some in 2012 and regarding what I Googled online regarding anti-1080 and, because somebody was in our house searching our computer and they can look up everything that we ever Googled.
"So they looked for everything we ever Googled and asked us especially what - there is a Googled entry on, I don't know, on 11 May 2011, and asked me what that was and why I did that."
He said police arrived at his and his wife's Golden Bay home this week and searched that property, as well as the couple's tea shop business and vehicles.
"I saw one car after another come into our property and I don't know how many policeman, maybe 10 or maybe more. They had three search warrants, for our business, for our house, and for our car, and that was really shocking for us."
Deputy Commissioner Mike Clement said police were speaking to a number of people in relation to the threats and some properties were being searched.
"That's a routine part of investigation, so if a person of interest becomes of significant interest to the extent that we can get a search warrant, then that's what we do."
Mr Clement would not say how many people were being spoken to for the investigation.