As New Zealand police debate whether officers should be able to use cannabis if it were legal, the Canadian police union supports officers smoking up - as long as they remain fit for duty.

"If it's legal, I don't think you should prohibit someone from consuming the substance," said Canadian Police Association president Tom Stamatakis, who is due to speak via video-link at the NZ Police Association annual conference in Wellington tomorrow.

The main theme of the conference is how to police cannabis if it were made legal for personal use in New Zealand. A referendum on legalisation by or at the 2020 general election is part of the confidence and supply agreement between Labour and the Greens.

The association is not taking a stance on whether it should be legalised, but believes it is important to discuss how to be prepared in case there is a "yes" vote.

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One of the complex issues is how to measure impairment from cannabis use, not only for drivers and workers, but also for on-duty police officers who want to consume cannabis legally.

Cannabis will be legal in Canada in a week, and Stamatakis said the Canadian Police Association does not support banning officers from using it.

"Our expectation is that police officers would be fit for duty when they report for work. It's the same expectation when it comes to alcohol or prescription drugs.

"If someone's at work and you feel they are impaired, you would enter an investigation based on certain symptoms - the same as for alcohol. If I could smell alcohol and they were unsteady on their feet, you could demand a breath test."

But despite the union's stance, Stamatakis said there were different rules across the 200 police forces in Canada's 10 provinces; some had an outright ban for police officers, even while off-duty, while others wanted a 24-hour or 48-hour prohibition before work.

"There is no consistent approach to this, and no definitive evidence to say what should happen. If you and I smoke a joint, it may affect us in a completely different way."

Stamatakis said police in jurisdictions where cannabis was legal should have a consistent and reasonable test for impairment, but knowing where to draw that line was difficult.

Police Minister Stuart Nash told reporters this morning he would be "horrified" if stoned police showed up for work.

NZ Police Association president Chris Cahill agreed, but noted that a person could consume cannabis and not be impaired.

"If cannabis is legal, cops can use it too," Cahill said in his opening remarks to the conference this morning.

"The consequences of THC [the psychoactive ingredient in cannabis] being detected in a drug test following a critical incident, such as a police shooting, are extremely serious for us. If cannabis becomes legal, our members need to know exactly where they stand on recreational use themselves."

For drug-affected drivers, Canadian law allows up to a certain amount of THC in the bloodstream, but Cahill called the amount "unscientific and arbitrary".

Stamatakis agreed, but he was hopeful that legalisation might spur new research and more evidence-based guidance.

He also said there were inconsistent age limits - some at 18, others at 21 - in different Canadian provinces, and while federal law in Canada allows for four home-grown plants, it will be impossible to enforce.

"There's no way we would have the capacity to police that in any way at all. It relies on individuals to follow the law, and if they're not, it relies on other citizens' reporting.

"But at what point do you take enforcement action? Do you deploy police resources to respond to someone growing six plants?"

But Stamatakis said the new framework in Canada was an improvement and made clear what is and is not legally permissible.

"Our existing framework was prohibition, and it wasn't working."