New Zealand's "resilient" demand for hard drugs is laid bare in a new report from the United Nations.

The UN agency tasked with upholding international anti-drug treaties outlines big increases in seizures of cocaine, heroin and ecstasy while noting high availability of methamphetamine.

And with a binding referendum due next year, the International Narcotics Control Board (INCB) offers a hardline stance on cannabis, saying the drug represents a "significant challenge to health and wellbeing, particularly among young people" and legalising it "contravenes the international drug control treaties".

Released late last night , the INCB's annual report details its efforts to "identify and predict dangerous trends" and suggest "necessary measures".

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The section on Oceania includes data about drug seizures and use in New Zealand. Although it only notes changes between 2016 and 2017, researchers here suggested those figures chimed with longer-term trends.

Over that period, the amount of cocaine seized tripled. While the number of busts rose less sharply, that indicated traffickers were trying to move bigger amounts at a time.

The amount of meth, or P, seized dropped sharply, although the 2016 figure contained the largest single bust in New Zealand history, a haul of around 500kg found on Ninety Mile Beach.

But the price of meth fell from $600 per gram in 2016 to $500 a year later, indicating a high availability.

Less amphetamine and so-called ecstasy "mimics" — drugs with similar effects — were found. The amount of meth precursors — drugs such as ephedrine used to cook P — intercepted was at its lowest level since 2012. That meant, the report concluded, suppliers were importing the end product instead.

Drug Foundation executive director Ross Bell said that showed the "market is very resilient to law enforcement efforts".

"If we push in one part of the market then it responds in some other ways. Fundamenally, there will always be a supply of drugs as long as there is a demand. If New Zealand wants to get serious about its drug problem it needs to put more resources into trying to reduce that demand."

Cannabis remains popular, particularly among the young, with an estimated 445,000 users in New Zealand.

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The INCB said it was behind a rise in the number of drug-related deaths between 2013 (178) and 2015 (254). It was thought synthetic drugs had contributed to that increase. Earlier this week, the Coroner said more than 65 people were thought to have died from synthetic drugs in New Zealand.

The report strongly opposed allowing cannabis for non-medical use. While not commenting on New Zealand's referendum directly, it said such moves posed significant health threats including, in the long term, dependence and the prospect of young people who use daily developing depressive symptoms and suicidal ideation.

Justice Minister Andrew Little rejected that view.

"The INCB was set up to promote prohibition. There is now a wealth of evidence showing that this simply has not worked and does not prevent the health risks they say they want to minimise.

"Many other countries are looking at alternative approaches to minimise the health risks of drugs. This is the debate New Zealand is about to have as we head towards the referendum."

Bell was also critical.

"They are unable to think about alternatives. Their mandate is to be guardians of the drug treaties. Their DNA doesn't allow them to see any positives.

"We've been doing this thing for 40 years and New Zealanders are still among the highest users of drugs in the world. Perhaps it's time to do something differently. I think the referendum, while it's not the sole answer to NZ's drug problem, could be a circuit-breaker.

"Creating a regulated market could be one of those ways that we start trying to treat drugs as a health issue and start focusing on the demand side of the equation."

The INCB describes itself as an independent and quasi-judicial control organ. Its 13 members are elected by the UN Economic and Social Council and serve in a personal capacity, not as government representatives.