An elderly Wellington woman - believed to be one of at least six euthanasia supporters visited by police - says an officer's door-knock was an attempt to intimidate her.
Police will not comment on the operation, but campaigners say people have been visited by officers for attending a meeting of euthanasia group Exit International, held earlier this month.
Wilhelmina Irving, who has been a long-time supporter of euthanasia, was at the meeting and said she was visited last week.
"It was a policeman, but he was in plain clothes," she said.
"They had the full membership list, they knew where the meeting had been held, they knew what was said - they knew it all," she said.
"It was a frightening tactic, I think."
Talking about the choice of life or death was fairly innocent, Mrs Irving said, and she said she did not take kindly to bullies.
A letter from the police listed support numbers that Mrs Irving could call.
Exit International head Philip Nitschke said he was aware of six members who attended the meeting on 2 October being visited by police.
"The police generally come along saying they're concerned about the individual's wellbeing, concerned about the situation or state they're in," he said. "Then of course they go on to say they're investigating people that attended that meeting, and whether of not they're going to be at some risk."
Dr Nitschke said the police's visits had frightened members, and he did not understand why officers were making them.
"It's very unsettling, and given there's an inquiry going on in New Zealand into issues of end-of-life choices, it's a very troubling development," he said. "It has the effect of chilling and intimidating many of our members."
Wellington City Area Commander Inspector Chris Bensemann said he was acutely aware of the interest in the sensitive nature of the investigations, but due to broader responsibilities to the Coroner, the police were not able to comment.
Earlier this week, an elderly Nelson campaigner, Patsy McGrath, said the police came to her house with a search warrant and seized a helium balloon kit.
The police also confirmed a Lower Hutt woman had been charged with importing a class C drug.