Police have been accused of "decriminalisation by stealth" after a study showed cannabis possession arrests have halved in the last 18 years.

A Massey University research centre report shows despite the number of users remaining constant, arrests for cannabis possession since the late 1990s have fallen.

The Government says its policy is anti-cannabis and anti-decriminalisation, but the research shows there were 454 arrests for every 100,000 people in 1998, but only 227 by 2006.

Labour Justice spokesman Charles Chauvel said police had recognised the current approach to cannabis was failing, and had implemented changes in spite of the law. "It's pretty much decriminalisation by stealth."

He said a recent Law Commission report recommended cannabis be treated more as a health than a criminal issue, but Parliament "failed to act".

At the time, Associate Health Minister Peter Dunne said the Government would not adopt the commission's recommendation of a three-warnings approach to cannabis.

Chauvel said he agreed with the police approach, but would "prefer it was under the supervision of Parliament".

Police national crime manager Superintendent Rod Drew denied police had gone "soft on cannabis" and said the figures were "ancient history".

Since 2008, when the study finished, there had been more arrests, police had a number of non-judicial avenues to resolve crimes, and officer discretion was a crucial part of policing.

The study's lead author, Dr Chris Wilkins, said about 15 per cent of the population used cannabis over the length of the study, but that fell to 13 per cent in 2006.

"The fact it's going down without a law change sets up a conundrum. It could be a generational thing or a gradual change in attitude. Maybe police are thinking it's just not worth it."

He said during an informal approach to police they suggested the fall was "most likely a re-prioritisation of enforcement efforts to methamphetamine".

Retired police officer and now drug policy consultant Mike Simmons said the figures suggested police were using more discretion – "finding a person with a tinnie, giving them a warning and not taking it any further". But during his 19 years in the police, he had never been told not to pursue cannabis possession.

Police Minister Anne Tolley said she did not tell police how to enforce the law. "But this Government believes there is no place for drugs, or the supply and manufacture of any kind of drugs in our communities. There is certainly no change in our focus."

But she added: "I expect police to act to keep our communities safe, which includes alternative ways of dealing with low-level offending to ease the pressure on the justice sector."