It would be frightening if police had used a parliamentary inquiry into euthanasia as the basis for setting up a "dodgy" breath testing checkpoint to identify euthanasia supporters, Labour leader Andrew Little says.
Questions are still being raised about what prompted police to set up the checkpoint near an Exit International meeting last month as part of their investigation into a suspected euthanasia death.
Act Party leader David Seymour is expected to open Question Time with the issue on Tuesday, asking Police Minister Judith Collins if the public are right to be concerned about police using roadside breath testing to collect personal information for unrelated investigations.
Mr Little earlier said he agreed with those who said it was an abuse of power.
"Even if it was about investigating people who they thought might vaguely be linked to a specific death in Lower Hutt they simply should not have done it," he said.
Asked whether police may have been reacting to the current parliamentary inquiry into assisted dying, the result of a petition by Labour MP Maryan Street, was "frightening".
"That is not the way our constitution is drawn - the police are there to uphold the laws, investigate them and act appropriately and properly on the basis of the information they receive.
"They should not be setting up dodgy checkpoints for ostensibly one purpose but using them for another. That is never acceptable."
The Government says the issue is a matter for police, who said they were acting in good faith.
The matter has been referred to the Independent Police Conduct Authority.