Rolf and Ute Kleine found out their house was bugged when Rolf was a suspect in an operation to find a blackmailer who threatened to spike milk powder with 1080.

A Golden Bay couple suing police for being wrongly targeted in a 1080 blackmail investigation have been shocked to discover their home was bugged.

Rolf and Ute Kleine, who run teahouse and bakery Takaka Infusion, obtained police documents through their lawyer in November that reveal their house was bugged with a surveillance device 20 days prior to a police search of their home in 2015.

The Kleines have lodged a statement of claim with the Nelson High Court and are suing police next year.

They say both searches and their warrant applications were unlawful and illegal, and they had been inaccurately profiled by police seeking those responsible for a threat to Fonterra to put 1080 pesticide into baby milk formula.

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In one of the police documents the couple obtained, Senior Sergeant Aaron Pascoe wrote that "very little" conversation could be picked up through the surveillance device planted in their home.

"I believe that the lack of evidence to date from the Surveillance Device warrant is either due to the poor quality..., or an indication that Rolf Kleine has acted alone (so he has not spoken to his wife about what he has done), or both," he wrote.

Rolf Kleine said the whole thing "keeps getting worse and worse".

"Police didn't find anything, not even a little bit against us, and they just kept going and kept going with no evidence at all," he said.

Ute Kleine said the couple used to joke that police might have bugged their home, but when it was revealed they really had, they were left reeling again.

"It's different if it's your phone than if it's really your private room, and they can listen to everything you say and do," she said.

"When I am at home, sometimes I have this feeling, 'is there somebody listening to me?' It's not a good feeling at all. Other times I wonder how many times did they actually enter our house; it must have been at least three times."

In response to the couple's claims a police statement said: "While Police have previously responded publicly on matters raised by these individuals, we are unable to provide further comment at this time while civil litigation is ongoing."

The Kleines are open anti-1080 activists. Rolf Kleine became a suspect in the operation to find the blackmailer after previously writing letters to Fonterra with concerns about 1080 being dropped near dairy farms.

The police search of the Kleines' house in March 2015 and the hours of questioning that followed left the couple traumatised.

They later learned of an earlier secret search in which police took hair samples, used dental floss, toothbrush swabs, and copied their electronic devices.

Another document obtained by the Kleines reveals Fonterra had employed private investigators, who emailed police to say that "he [Rolf Kleine] is not the same author as the...letter."

Further documents also reveal Rolf Kleine was a suspect of another anonymous 2011 letter to Fonterra, in which the blackmailer threatened to release bovine disease into New Zealand if it did not decrease "dirty dairying" practises.

It said Fonterra received another letter six days later retracting the threat and apologising for the over-reaction.

Last year, Auckland businessman Jeremy Hamish Kerr, 60, was sentenced to eight and half years' jail on two counts of blackmail for writing the 1080 threat.

Kerr owned a poison company that made a rival pest-control called Feratox.

Rolf Kleine said police would have used grounds of "terrorism" to get the warrant for the secret search, otherwise they wouldn't have obtained it under privacy issues.

"They wanted to intimidate us because we are anti-1080, and they did."

TIMELINE