Police broke the law when they used a breath-testing checkpoint to identify people who had been to a pro-euthanasia meeting in Lower Hutt, the police watchdog says.
Act New Zealand leader David Seymour has called for the resignation of police commissioner Mike Bush.
The meeting by Exit International, a group lobbying for the legalisation of euthanasia, took place in October 2016, before a police checkpoint near it then collected names and addresses of those who had attended.
Police had been monitoring the meeting as part of an investigation into the death of an elderly woman, who ingested pentobarbitone, a controlled drug used to euthanise animals.
When they overheard a discussion about how to import the drug, they feared meeting attendees were at risk of harming themselves and set up the checkpoint to identify them.
However, an Independent Police Conduct Authority report released yesterday found this was unlawful.
While police are permitted to stop motorists to enforce transport laws, the Lower Hutt checkpoint was illegal because it was used for a different purpose, authority chairman Judge Colin Doherty said.
"Police should have recognised that they had no power to stop vehicles in these circumstances. It was an illegitimate use of police power," he said.
Privacy commissioner John Edwards backed the findings, and said police had abused the public's trust.
"There is an expectation that [police] will follow the law and their own policies at all times."
He said police then caused further harm by using the information from the checkpoint to "unlawfully" visit people in their homes, and question them about a "socially and politically sensitive subject".
This caused some to feel "uncertain about their ability to speak freely and anxious that more visits would follow", he said.
However, he and Judge Doherty acknowledged the police had acted with good intentions.
Police assistant commissioner Bill Searle accepted the findings but said his staff acted "to protect life and did not intentionally break the law".
Police Minister Stuart Nash declined to comment and referred questions to the police.
Mr Seymour, sponsor of the End of Life Choice Bill which is being considered by Parliament, said the police response to the IPCA report was unacceptable.
"Requiring police to follow the law is what separates a free society like New Zealand from becoming a police state," he said.
"The police commissioner had a chance to take responsibility for his officers breaking the law and, having ducked the issue, is not fit for office."
In February, 67-year-old Susan Austen was found not guilty by a High Court jury of aiding Annemarie Treadwell to commit suicide in 2016, using the imported drug pentobarbitone.
