Any change to drug laws has got to take supply out of the hands of criminals to reduce the harm to society, the Police Association says.
Association president Greg O'Connor told Morning Report that any liberalisation of drug laws needed to recognise that you cannot decouple drug use from the dealers.
He distinguished between the approaches taken in the Netherlands and the US state of Colorado, saying only the latter had tackled supply. He argued that the simple tolerance adopted in the Netherlands did nothing to regulate drug dealers.
"If you are going to liberalise our drug laws, you have to have a look at Colorado which is probably a better model than the Netherlands" he said.
However, Mr O'Connor refused to say whether or not he supported the adoption of a Colorado-style approach in New Zealand.
He said he had looked first-hand at different approaches and warned against copying the Netherlands - where drug use was allowed but criminals still controlled supply.
"So you get the worst of both worlds, where you get all the increased use, but you don't decrease the damage to society. Whereas if you look at Colorado, they have made conscious attempts in legalising the whole process, from seed to weed."
He said society would only be better off if organised crime was weakened.
The government has signalled that it is open to taking a more tolerant approach to minor drug offences.
Associate Health Minister Peter Dunne has said he was not sure New Zealand's drug law was still fit-for-purpose and he wanted drugs to be viewed as more of a health issue.
Mr O'Connor is going to Portugal next month, where all drug use was decriminalised in 2001 and drug deaths were very low.
De facto decriminalisation?
Meanwhile, the Drug Foundation said a fall in the number of people convicted for minor drug crimes was virtually de facto decriminalisation.
Minor drug convictions among 17- to 25-year-olds are down from 2500 in 2007 to under 1000, most of them for cannabis.
The Police Association said the drop in convictions was partly due to the government telling the police in 2010 to bring 19 percent fewer people into the criminal justice system.
Ross Bell of the Drug Foundation said the government should step up.
"It almost points to a de facto decriminalisation anyway. The politicians should cement that policy change in law, to reflect the reality that it is not worth using police resources to go after minor offenders."
The police stressed there was no target for reducing the number of minor drug convictions.
Acting National Crime Manager Tim Andersen said arrests for all sorts of minor crime had dropped under a smarter policing approach that used pre-charge warnings much more.
"We don't have goals around this per se. Each case is a case-by-case basis," he said.
"We realise you can't arrest your way out of a social harm situation. So in terms of that statistic, yes, it's down, but the reasons are sound."
Mr Andersen said this was not a form of de facto decriminalisation as illicit drug use and possession were still offences.
A new poll released yesterday suggested nearly three-quarters of New Zealanders support allowing marijuana to be used for medical purposes.
But Justice Minister Amy Adams said there was very little support for any change when this was looked at last time, and Health Minister Jonathan Coleman said he was not in favour of decriminalisation.