In New Zealand, cannabis is classified as an illegal drug under the Misuse of Drugs Act 1975. Its possession, use and supply are subject variously to penalties ranging in severity from fines to many years of imprisonment. On Wednesday, the Helen Clark Foundation released a report which sets out the case for legalising and regulating cannabis. New Zealanders have the opportunity to vote for that in a referendum next year.

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Clearly, the prospect of invoking criminal sanctions has had little impact on people’s behaviour. Evidence from longitudinal studies carried out in New Zealand indicates that by the age of 25, 80% of New Zealanders will have tried cannabis at least once. Put simply, prohibition-based policy approaches have not eradicated and will not eradicate cannabis consumption and supply in New Zealand or anywhere else where its use is established.

On the weighted score for harms of drugs in the United Kingdom developed by scientist David Nutt, former chair of the Advisory Council on the Misuse of Drugs, and colleagues, and published in the Lancet in 2010, cannabis is assessed as immensely less harmful to those who use it, and to others, than is alcohol. Various other rankings concur that cannabis use is significantly less problematic for individual health than either tobacco or alcohol.

A 'yes' vote in the 2020 referendum will be positive for social justice and equity

The classification of drugs pursuant to the international drug conventions, however, has been based more on cultural and political factors than on scientific evidence. Those classification judgments have scarred the lives of countless millions of people around the world who have been caught up in the criminal net cast over what is deemed to be illegal drug possession, use and supply.

It is not surprising, therefore, that a growing number of jurisdictions have been moving away from the prohibitionist approach promoted by the international conventions. Canada, Uruguay and several states in the United States have legalised the possession, use and supply of cannabis. Other jurisdictions have decriminalised personal possession and use, but not supply. The proposal to be put to the New Zealand referendum will be to legalise and regulate, with the exact form that takes yet to be determined.

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It will be important in legalising and regulating not to create another “big tobacco” or “big alcohol”. Both those industries have fought, and continue to fight, efforts to reduce the harm they cause. Those efforts include regulation of access, product promotion, and taxation aimed at disincentivising use. While on an objective assessment, cannabis is less risky overall to public health, it is not without risk. Just as harm reduction is applied with respect to other products and behaviours, from the legal drugs to gambling, road and food safety, and much else, so it should apply to legalised cannabis, accompanied by upfront public health awareness education.

A recent amendment to New Zealand’s Misuse of Drugs Act has directed police only to prosecute those using drugs when there is a “public interest” in doing so. The government has been clear that it wants to take a health and wellbeing-based approach to those who use drugs. The emphasis on police discretion, however, means that prosecutions for cannabis use and possession would still remain possible, and prosecutions for supply would continue as they do now.

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Without legalisation, major ethnic disparities in arrest, prosecution and conviction for cannabis-related offending are also likely to persist. Indeed, current practice in the New Zealand justice system with respect to drugs is profoundly unjust to the Indigenous people, Māori. They bear a disproportionate burden of the prosecutions, convictions and custodial sentences handed down for drug-related, and indeed other, offending. Māori make up around 15% of the population. Yet Māori aged 17 to 25 make up 37% of all convictions for drug possession. The current government is keen to reduce the numbers of prisoners in New Zealand and within that total to reduce the proportion of Māori incarcerated, to reflect their proportion of the population. Drug law reform, including the legalisation of cannabis, helps meet both those objectives.

The time has come for New Zealand to face up to the widespread use and supply of cannabis in the country and to legalise it and regulate it accordingly. No useful purpose is served by maintaining its illegal status. A “yes” vote in the 2020 referendum will be positive for social justice and equity, contribute to reducing the country’s excessively large prison population, and enable those health issues associated with cannabis to be dealt with upfront. These are the reasons why I support legalisation.

• Helen Clark is a former prime minister of New Zealand and member of the Global Commission on Drug Policy