Police overstepped their authority by setting up a 2016 breath testing checkpoint near a Hutt Valley pro-euthanasia group meeting, an independent investigation has found.
The meeting was being held at the home of Susan Austen, and police set up a checkpoint nearby to gather the names and addresses of many of the attendees.
Susan Austen, pro-euthanasia campaigner and co-ordinator of Exit Wellington, was under investigation for allegedly aiding in the suicide of Annemarie Treadwell who had died in June 2016.
The Independent Police Conduct Authority has ruled Police were not justified in establishing the checkpoint to target the Exit International meeting.
Privacy Commissioner John Edwards said police, "unlawfully and unfairly collected personal information, harming some of the people affected".
Operation Painter was established by Police to monitor the pro-euthanasia group.
In the Privacy Commission's own report, they say the way police obtained the information breached principle 4 of the Privacy Act 1993.
"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," Mr Edwards said.
Police used the information gathered at the checkpoint to conduct welfare checks on approximately 15 of the attendees and argued they had concerns for their immediate welfare.
Not all attendees believed the visits were just about welfare, telling the IPCA, "...it seems odd and I think they were there for other reasons than just about my state".
While the complaints of attendees were heard, the IPCA ruled Police's subsequent welfare checks were appropriate, but said in future police should consider using another organisation to provide support.
Susan Austen was in February found not guilty of aiding a suicide and guilty of importations of the drug pentobarbitone.
Austin is on bail while awaiting sentencing on May 11.
Police said in a statement that they accepted the findings, but that none of their officers had intentionally breached the law.