The decision to ban cannabis was an accident of history.

There was no careful root and branch review of the evidence. Instead, Australia was represented at a League of Nations meeting in Geneva in 1925 where delegates from several countries decried the dangers of cannabis. As Robert Kendell outlines in his book Cannabis Condemned:

"A claim by the Egyptian delegation that [cannabis] was as dangerous as opium, and should therefore be subject to the same international controls, was supported by several other countries. No formal evidence was produced and conference delegates had not been briefed about cannabis."

Accordingly, the Commonwealth wrote to the states after the meeting instructing them to prohibit cannabis.

This is the quicksand upon which the mighty edifice of cannabis prohibition in Australia was constructed.

Once enacted, repeal — or even a careful review — of benefits and costs of cannabis prohibition became increasingly difficult.

Pot convictions have 'adverse social consequences'

Cannabis arrests have accounted for the largest proportion of illicit drug arrests in Australia. In 2015-16, of the two million Australians who use cannabis every year there were almost 80,000 cannabis arrests, a 6 per cent increase from the previous year.

Of these arrests, the overwhelming majority (90 per cent) were consumers while the remainder (10 per cent) were providers. Yet in 2017, 92 per cent of drug users reported in a national survey that obtaining hydroponic cannabis was "easy" or "very easy" while 75 per cent reported obtaining bush cannabis was "easy" or "very easy".

In the 1980s, according to the United Nations Office on Drugs, South Australia adopted a Cannabis Expiation Notice (CEN) scheme while Western Australia retained a harsher approach:

"A comparative study of minor marijuana offenders in South Australia and Western Australia concluded that the more punitive prohibition approach had little more deterrent effect upon marijuana users than the CEN scheme. The adverse social consequences of a marijuana conviction were, however, seen to outweigh those of receiving an expiation notice. In fact, a higher proportion of those apprehended for marijuana use in Western Australia reported problems with employment, further involvement with the criminal justice system as well as accommodation and relationship problems."

It is difficult to find reliable estimates of the cost of cannabis prohibition in Australia and an even greater challenge to identify benefits from banning the drug.

Supporters of cannabis prohibition frequently assert with great confidence that punitive approaches reduce consumption while a lenient approach increases use. Yet credible evidence to support this contention is unimpressive.

Drug policy has surprisingly little effect, if any, on consumption patterns but does produce serious harm.

A study comparing residents of more liberal Amsterdam and more punitive San Francisco using the same methodology found less illicit drug use (including cannabis) in Amsterdam and a far greater likelihood that San Francisco residents were also offered heroin, cocaine or amphetamine on the most recent occasion of trying to buy cannabis.

The harms resulting from cannabis prohibition are far greater than the harms resulting from cannabis itself as former US president Jimmy Carter observed:

"Penalties against the use of a drug should not be more damaging to an individual than the use of a drug itself; and where they are they should be changed. Nowhere is this more clear than in the laws against the possession of marijuana."

People queue to buy recreational marijuana in Colorado State, where tax from cannabis has funded road and school repairs. ( Reuters: Rick Wilking )

Cannabis tax revenue could fund treatment

Taxing cannabis would provide much-needed revenue for government.

Sorry, this video has expired Greens announce plan to legalise cannabis

Colorado state in the US began taxing and regulating cannabis in 2014 and has used the revenue generated to rebuild old school buildings and repair roads. California is using this tax to rejuvenate areas badly damaged by the War On Drugs.

Australia could allocate these funds to improving and expanding alcohol and drug prevention and treatment, an area governments usually find difficult to fund properly.

Regulation would enable governments to mandate plain packaging, like we have for cigarettes. Packages should provide health warnings, help-seeking information and consumer product information (including content of psychoactive ingredients and their concentration).

Proof-of-age restrictions on sale, similar to arrangements for alcohol, could help reduce access to cannabis for underage Australians.

Government-commissioned guidelines for less risky alcohol consumption have existed for decades. Regulating cannabis would enable government to start commissioning guidelines for less risky cannabis consumption.

All advertising of cannabis should be banned from the outset. If possible, donations to political parties from any cannabis industry should, if possible, be prohibited.

Regulation could ease strain on law enforcement

Regulating cannabis would also mean that law enforcement could concentrate on responding to far more serious crimes — especially violent crimes.

A WA police officer pulls up a cannabis crop. ( Supplied: WA Police (file photo) )

Regulating cannabis would also reduce some of the cost of customs, police, courts and prisons and some of the income of major league black-market cannabis suppliers.

A poll by Essential Media found in 2016 that 55 per cent of Australians support taxing and regulating cannabis, with at least 50 per cent of the following groups supportive: men, women, ALP voters, Greens voters. 47 per cent of LNP voters favoured taxing and regulating.

Four polls in the US in 2018 have found over 60 per cent support for taxing and regulating cannabis, with over 50 per cent of Republican voters now in support.

Cannabis taxation and regulation is an idea whose time has come.

Like the debate about same sex marriage, the idea will seem strange to some in advance but once the reform has been completed we will wonder why it took us so long.

Dr Alex Wodak AM is the president of the Australian Drug Law Reform Foundation.