Police are vigorously attacking drug suppliers but arresting fewer people for possession - though they insist there has been no change in policy.

The release of fiscal year crime statistics last week painted a strange picture of a police force pursuing a drug policy applauded by left-leaning commentators - light on users and hard on suppliers - at the same time as denying any change has taken place.

Police statistics for drug use and possession were the lowest for 18 years, Statistics New Zealand reporting 8771 cases in 2011-12. About three-quarters of that number were for possession of cannabis, also at its lowest level for 18 years.

Conviction statistics, released last week, showed a similar decline: the 5685 cases for drug possession that went before the courts were the lowest since 1981-82, when New Zealand had a quarter less people. While possession arrests declined, apprehensions for sale or supply of all drugs leapt from 723 in 2010-11 to 2031 in 2011-12.

The numbers confirm a trend identified by Massey University's Shore research team in April that possession arrest rates had halved between 1998 and 2006 while rates of cannabis consumption stayed constant.

That report was interpreted as "decriminalisation by stealth" by the Labour Party's Charles Chauvel, who applauded the approach but said it would be better if it was supervised by Parliament through legislation.

Detective Inspector Paul Berry said there had been no policy change and he had never heard anyone say "let's stop doing this and do this now". He said he did not know what was driving the statistics down but suggested the figures could represent an increased focus on dealers and traffickers and an increased use of warnings so as not to "clog the justice system up".

Berry admitted that in the 1980s police probably would have responded to a tip that cannabis was being used at a house but "you'd have to be pretty desperate to do that now - we've got better things to do".

Former police drug detective and now drug policy consultant Dale Kirk said a "gradual liberalisation of society" had meant decriminalisation had "de facto happened without anyone making a big fuss about it".

Researcher Dr Chris Wilkins thought arrest statistics reflected a change of police focus to methamphetamine. "I think this is a good story. If you're going to have drug enforcement it should concentrate on the drug that causes the most harm, and that's methamphetamine."

- © Fairfax NZ News