Legalising marijuana for personal use is not a silver bullet and won't see the end of gangs or cannabis-related crime, nor free up police resources to tackle other crime, the Police Association has heard.

The warning comes from police intelligence practitioner and cannabis researcher Carrie Drake, who was one of several speakers at the second day of the association's annual conference in Wellington today.

The conference theme is about policing 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.

Drake interviewed frontline officers in jurisdictions where cannabis was legal or decriminalised, including US states and the Netherlands, as part of research while at Massey University.

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She found that legalising cannabis is often viewed as a panacea, but cannabis-related social harm existed regardless of the drug's legal status, gangs continued to cause social havoc, and police continued to be called to cannabis-related incidents.

Drake said a US officer told her: "We just have not seen all the wonderful promises that were made to us."

When association president Chris Cahill asked members for a show of hands from those who expected legalisation to allow resources to be deployed elsewhere, no one raised a hand.

Officers in some US states also told Drake that there seemed to be an increase in drug-affected driving, but testing for impairment was unenforceable.

The difficulty of testing for impairment - not just for drivers, but also workers, including on-duty police officers - has been a major topic of debate in the conference.

Drug Foundation executive director Ross Bell, who chaired today's discussion, highlighted the example of Air NZ, which won a landmark case allowing for random drug-testing in safety-critical workplaces.

"But today, drug-testing is a tiny part of what they do," Bell said.

"They've gone down the path of wellness, and building a culture where it's not tolerated to come to work impaired. You can dob in your mate and they will get support, rather than punishment."

Massey University Associate Professor Chris Wilkins said the cannabis issue was not a binary choice between legalisation and prohibition.

"There are at least 10 options. If it's a commercial market, all the profit goes to shareholders and the aim is to grow the market. There could be no commercial market at all, or a not-for-profit option, where a good proportion of all the money spent on cannabis would go back to the community for things like drug-treatment and drug-prevention.

"We already have good examples of that like gambling trusts or alcohol-licensing trusts. If we had that for alcohol, 40 per cent of all money spent on alcohol - instead of buying Ferraris and big boats - would go towards alcohol and drug treatment, and other activities to mitigate the problems of alcohol."

Green Party MP Chloe Swarbrick told the conference that the referendum would "probably" be at the 2020 election, though a spokeswoman for Justice Minister Andrew Little said the timing was still to be decided.

Swarbrick said the Greens wanted to have a bill in place by the time of the referendum, so voters will know what was at stake.