A police checkpoint targeting euthanasia supporters in Lower Hutt in 2016 was unlawful, the Independent Police Conduct Authority says.
The Privacy Commissioner has also condemned police actions, saying officers unlawfully and unfairly collected personal information and harmed some of the people involved.
Police have accepted they broke the law - but maintain they "acted in order to protect life and did not intentionally break the law".
The checkpoint on October 2 was set up to identify people who had attended an Exit International meeting that had just taken place.
Exit International is a pro-euthanasia group.
Police had been monitoring the meeting with a surveillance device as part of an investigation into the death of Annemarie Treadwell, who had ingested pentobarbitone, a controlled drug used to euthanise animals.
Head of the Wellington branch of the group, Susan Austen, was found not guilty last month of helping Treadwell commit suicide, but was found guilty of importing the drug.
Austen said she was pleased with the authority's findings.
"I think that people should be able to go to someone's home and have a totally lawful meeting and gathering and not have to have any fear that there may be repercussions."
The authority's report says that while police were monitoring the meeting they overheard a discussion about how to import the drug and ways to commit suicide.
"It wasn't until we heard [them] conveying all these possibilities to commit suicide . . . that we were of the view that the risk level had raised considerably," one officer told the IPCA.
The area commander believed the decision to set up the checkpoint was reasonable in the circumstances, the report said.
Police planned to provide welfare support to those identified, and visited a number of people for that purpose several days after the checkpoint happened.
Police power to stop vehicles is for the purpose of enforcing land transport legislation, and because the checkpoint in question was not for that purpose, the IPCA found it was unlawful.
"Police should have recognised that they had no power to stop vehicles in these circumstances", said chairman Judge Colin Doherty.
"It was an illegitimate use of police power that unlawfully restricted the right of citizens to freedom of movement."
However, the IPCA found the subsequent police welfare support visits did not breach the Privacy Act and were in accordance with operational policy and the duty of police to protect life and safety.
"The authority acknowledges that the welfare support visits generated stress and apprehension among some of those visited, but they were well intentioned and generated by a genuine concern for the wellbeing of those visited," Judge Doherty said.
One woman who was visited by police said the welfare visit "seemed patronising to elderly people".
"I mean, why should, just because one's 85, one be assumed to be kind of helpless and weak and pathetic and needing comforting police to come and pat you on the back?"
Another woman said she'd never had contact with police before and immediately thought someone in her family had been in an accident.
Police Assistant Commissioner Bill Searle said police accepted the checkpoint had been unlawful.
However the IPCA had acknowledged that the subsequent police welfare visits were in accordance with police policy and did not breach the Privacy Act.
"The authority also notes that staff sought advice from suicide prevention specialists, and that the staff conducting these visits were briefed to treat individuals with dignity, respect, empathy and professionalism."
Searle said police "fully recognise the sensitivities about euthanasia, however we take no moral position about this issue".
Austen's lawyer Dr Donald Stevens QC said "one of the more troubling features of the report" was that the acting Area District Commander and the Area Commander had not recognised what their staff were planning to do was unlawful and stopped them.
He blamed the under-resourcing and lack of training of police for the issue.
Despite the authority's findings he questioned whether the police's welfare visits were appropriate.
"Anyone who was in court and listened to the audio of the bugged conversation from the Exit meeting would not think that suicide was imminent... as the police seem to think. It was a collection of people who were seeking to ed and inform themselves about their possible future options should their health should take a turn for the worse. "
Privacy Commissioner John Edwards said the visits from police left people feeling uncertain about their ability to speak freely, and anxious that more visits would follow.
He said the way the information was collected breached the Privacy Act.
"Police used an unlawful checkpoint to take advantage of the public's trust in them and collect information from people who were not legally required to provide it.
"Police approached them after unlawfully collecting their information, and questioned them about a socially and politically sensitive subject. It is fair to say that the actions by the police officers caused those complainants harm," Edwards said.
He acknowledged that police believed the meeting attendees were at risk, and said apologies from police and an undertaking to delete the information collected at the checkpoint were appropriate resolutions to the complaints.