UPDATED 8.03am Tracking euthanasia campaigners through a police breathalyser stop, is being labelled an abuse of power by ACT leader David Seymour.

LISTEN ABOVE: ACT Party leader David Seymour spoke to Rachel Smalley

Police have admitted using the checkpoint in Lower Hutt earlier this month, saying they wanted to gather the identities of those attending an Exit International meeting, who may have been considering suicide.

MORE: Police admit targeting pro-euthanasia campaigners in drink drive operation

David Seymour said officers are not given powers for one purpose, so that they can abuse them for another.

"Stopping people for a breath test in order to get names of a group of people that have been at a meeting which is semi-political - that's the kind of thing you'd expect in Putin's Russia, but not in John Key's New Zealand."

Seymour said it's treating members of an advocacy group as if they're gang members.

"How did they decide that some little old ladies trying to work out how to end their lives peacefully if they're very ill, was a bigger priority for New Zealanders' safety than burglaries, which are out of control?

"Frankly, [it's] a little bit creepy and very un-Kiwi, because one thing about being in New Zealand is that we actually believe the police are beyond reproach and act with the greatest integrity at all times."

Mr Seymour said the meeting could have been politically sensitive given that some of the attendees were submitting to a parliamentary inquiry on assisted dying.

He told Rachel Smalley that raises questions.

"Did they seek any legal advice? Did they inform the minister of police that they were getting involved in something that could be seen as politically sensitive, and if they did, does the minister of police think that this is acceptable."

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."

Police initially defended their actions, but have now referred themselves to the Independent Police Conduct Authority.

A spokeswoman for police minister Judith Collins said the case is an operational matter.