Neither of the complainants were involved in the police operation in Lower Hutt, in which police used a drink-driving checkpoint to collect details on people attending an Exit International meeting.
Right-to-die campaigner and former Labour MP Maryan Street also plans to make a complaint to the IPCA.
Christchurch-based criminal barrister James Rapley said the case was a clear abuse of power and appeared to go against arbitrary detention and freedom of assembly rights.
Police powers in the Land Transport Act to stop people relate strictly to driving-related concerns, he said, and could not be used for any reason.
Rapley said the law also prevented police from asking for people's names and addresses unless they were checking whether a person had a driver's licence.
He cited the case of Hells Angels gang member Mark Ghent, who had a charge of obstructing police thrown out after he was arrested for refusing to give his name and details to a police officer.
"That's what they're really doing here. And they're asking normal people, not gang members, and who are entitled to go to a meeting."
Any potential search warrants or charges which resulted from police actions at the checkpoint were likely to be thrown out in court, he said.
Rapley helped to get charges thrown out in another case which involved a police ruse, albeit one in which an undercover officer was used. Drug charges against Red Devils Motorcycle Club president Roger Patrick and associates were dropped after police were found to have faked an arrest of an undercover officer to boost his gang credentials.
He said the checkpoint case threatened to damage public perception of police officers.
"Police need to be worried about this, because it is going to erode public confidence in them."
Street, who is president of the Voluntary Euthanasia Society,said the operation raised questions about political interference, with an inquiry into assisted dying currently before parliament.
It appeared the operation had come out of an inquiry originally conducted on behalf on a coroner looking into a suspected suicide, she said.
"It raises questions about unhealthy interference across jurisdictions," she said.
"Police have overstepped the line here and need to abandon the investigation immediately."
Ms Street questioned whether police had briefed the Solicitor General or asked Crown Law for advice before the operation.
Acting District Commander Inspector Paul Basham said the operation was carried out "in good faith and for good reasons", but there were concerns about the legal basis of the checkpoint.
Police Minister Judith Collins refused to comment, saying it was an operational matter and that she did not want to prejudice the IPCA investigation.
Exit International's Philip Nitschke said it's unprecedented, and answers are needed.
"It is really hard to understand that this idea is, somehow or other, police acting in the best interests of New Zealanders. I don't know what's behind this but it's very hard to make that line of 'we're doing this to keep New Zealanders safe.'"
Mr Nitschke said no other country has done anything like it.
"Strikes me as being something which certainly needs explanation.
Effectively, now it's surveillance of people who have every lawful right to meet," Mr Nitschke said.
Mr Nitschke said the group's looking into what legal avenues it has to challenge the police actions.
Barrister Michael Bott said just because someone was at the meeting doesn't mean they were wanting to commit suicide, or help someone commit suicide.
"It appears to be some kind of moral crusade, driven by someone on top, to stop people going to a public meeting to learn about exit and their goals."
Mr Bott said police used the breath-testing laws for something other than they're meant for.
"You've got a very limited power and it certainly seems to be an abuse of police at the checkpoints' powers."
