Dealing with legal recreational cannabis, including how to measure impairment among drug-affected drivers and even on-duty police officers, is the major focus of the Police Association annual conference, kicking off tomorrow.

New Zealand will have a referendum on legalising the personal use of cannabis by or at the 2020 general election, as part of the confidence and supply agreement between Labour and the Greens.

Police Association president Chris Cahill said the association does not have a position on whether personal use should be legal, but police have to be ready to hit the ground running if it is a "yes" vote.

"We're not saying whether it should or should not happen, just that we should be prepared. It's for New Zealanders to decide."

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Among the issues to be addressed are whether a "yes" vote would increase drug-dependency and harm to young people, and how to measure impairment from cannabis use, he said.

"You can have cannabis in your system and have no impairment issues. That's the challenge."

Personal use of cannabis is about to become legal in Canada next week, with police there raising questions about officers that could use it legally while working. Canadian policy says they can use cannabis and come to work as long as they are not impaired - but measuring impairment is complex.

Cahill said the issue was similar to the challenge of measuring impairment in a driver who had used cannabis.

Canada's driving laws allow up to a specific amount of THC, the psychoactive ingredient in cannabis, in your blood while driving, but Cahill called the amount "arbitrary and not scientific".

He said New Zealand would need to have something scientifically-based and ready to be implemented when the referendum went to a vote.

"We don't want to be chasing our tails with an increasing road toll six months after its legalised because we weren't able to test appropriately for drug-affected driving."

Last month a member's bill from National MP Alastair Scott to allow for random roadside drug-testing of driver's saliva for THC, ecstasy and methamphetamine was voted down.

Transport Minister Phil Twyford, speaking at the bill's first reading, said the bill was "implausible and impractical" and tested only the presence of drugs - not impairment.

But he said drug-driving was a serious problem, and the Government was actively working on the issue, Twyford said.

Currently it is the driver's duty to be in a safe state to drive, and police can drug-test the blood of a driver who fails an impairment test.

Cahill said the association would look hard at the cannabis experience overseas, and the conference will hear from Canadian Police Association president Tom Stamatakis.

Also speaking is police intelligence practitioner Carrie Drake, who studied social harm by interviewing police in countries where the cannabis is legally available.

Her study found that cannabis caused harm regardless of its legal status, including in road accidents and in the mental health of young people from disadvantaged backgrounds.