Police charges for use or possession of cannabis have fallen by 70 per cent in the past decade, latest data shows.

But the Ministry of Justice statistics, released today, also show that police charges more than doubled for P use or possession since 2013.

And last year, for the first time, police laid more charges for P use or possession than for cannabis use or possession.

The Drug Foundation says this would indicate that police are rightly focusing resources on more harmful drugs, but the thousands of people still convicted for using cannabis was still too high.

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The figures come as Parliament is considering the Misuse of Drugs Amendment Act, which proposes a hard line on synthetic drug suppliers, but a health-based approach to drug use and possession, including police discretion to pursue prosecutions only if they were in the public interest.

There will also be a referendum on the legalisation of marijuana in 2020 at the same time as the general election, though the Government is still working out the details of what the public will vote on.

The figures showed a drop in the number of instances where people were only convicted for cannabis - meaning they were not convicted of any other offences on the same day.

This dived from 2653 instances in 2009 to only 540 in 2018, though the decline was far sharper from 2009 to 2015. Only eight people were sentenced to jail terms for cannabis possession or use last year.

This contrasts with the number of police charges for methamphetamine use or possession, which more than doubled in the last five years to 3890 charges in 2018.

Drug Foundation executive director Ross Bell said it was right for police to focus more on harmful drugs such as methamphetamine rather than less-harmful ones such as cannabis.

But he said there were still thousands of people being convicted for cannabis use or possession every year.

"People are arguing that we shouldn't worry about drug law reform because we're not locking people up, but that's not the issue.

"People are still getting a criminal conviction for possession of a drug that many would say is a low-harm drug. That conviction is very problematic, particularly when you're talking about getting jobs.

"These are thousands of people, and they are largely younger and Māori."

About 2300 people were charged each year with cannabis possession or use since 2015, of whom about 1700 to 1800 were convicted.

Bell said it would be interesting to see how police would implement the proposal to only prosecute if it was in the public interest.

Part of that discretion, the bill says, would be about "whether a health-centred or therapeutic approach would be more beneficial".

"In what cases would no one benefit from a therapeutic intervention?" Bell said.

"You could potentially see thousands of people a year not only avoiding police charges, but getting a positive intervention."

Data was also released for offences for driving under the influence of alcohol or drugs.

The number of people convicted has risen by 4 per cent since 2017, but has almost halved from 30,458 people in 2009 to 16,664 in 2018.

In July 2018, alcohol interlock orders became mandatory for repeat drink drivers and offenders with excessively high blood/breath alcohol concentration.

This contributed to a five-fold increase in the number of people who received an alcohol interlock order in 2018 compared to the previous year (1947 people in 2018 and 354 people in 2017).