VIRGINIA TRIOLI, PRESENTER: When it comes to illicit drugs, there's none more popular in Australia than marijuana - more than one third of us have tried it.

One of them is Dr Richard Di Natale and, yes, he did inhale, as Barack Obama once said - that was the point.

The Greens' leader is now leading a push to legalise recreational cannabis in Australia as it already has been legalised in some other countries.

Now, under the proposal for a government regulated market, sales would be banned to under 18s, and there would be strict penalties for selling the drug without a licence.

But such controls are not enough to quell serious concerns from the medical profession and the Federal Government.

Geoff Thompson reports.

GEOFF THOMPSON, REPORTER: Love it or hate it - recreational marijuana use is illegal in Australia and almost 80,000 of us are arrested on cannabis charges each year - more than any other illicit drug.

Possessing small amounts of cannabis has already been decriminalised in South Australia, the Northern Territory, the ACT, and partially in WA.

It remains a crime in every other state but those being arrested are rarely hardened criminals, says Australia's former top federal policeman.

MICK PALMER, FORMER AFP COMMISSIONER: Even in quite an early part of my career I came to recognise that the people we were arresting were, in every sense, decent young Australian men and women who committed no other crimes and weren't likely to come to the notice of police for any other reason.

GEOFF THOMPSON: Now a radical proposal to turn cannabis into a legal substance - regulated like alcohol and tobacco.

RICHARD DI NATALE, GREENS LEADER: What we're proposing is to take the black market when it comes to cannabis, away from the drug dealers, the criminals, and to treat cannabis consumption as a health issue, rather than a law and order issue.

GEOFF THOMPSON: Mick Palmer is NOW supporting the Greens' push for a government-backed agency to become the nation's dope dealer in chief.

RICHARD DI NATALE: We need to get real about drug use in Australia. We need to get real about cannabis.

The National Cannabis Agency would licence the cultivation of cannabis, it would licence growers under strict conditions.

It would then act as a wholesaler, to purchase that product and it would then be sold through licensed retail outlets.

GEOFF THOMPSON: Annual sales of legal cannabis in the US are tipped to soon be worth more than $30 billion.

RICHARD DI NATALE: Places like Uruguay, the US, many states there, Canada about to legalise cannabis for adult use.

In New Zealand, the Labour Party will shortly hold a referendum on cannabis for legalisation.

This is happening around the world. It's based on evidence.

It's only a matter of time before it happens here in Australia.

GEOFF THOMPSON: But it won't be happening any time soon says the Federal Government which supports the Australian Medical Association's opposition to legalising cannabis use.

GREG HUNT, HEALTH MINISTER: As the Government of Australia, our job is protect the health of Australians.

This action by the Greens risks the health of Australians.

It should be rejected and they should withdraw it and we will oppose it.

Cannabis has very serious risks in relation to physical health and, in particular, to mental health.

GEOFF THOMPSON: Karl O'Callaghan has intimate experience of where cannabis consumption can lead.

KARL O'CALLAGHAN, FORMER WA POLICE COMMISSIONER: I have spoken to my son about this and he says that cannabis was probably his gateway.

GEOFF THOMPSON: The former Western Australia Police Commissioner's son became addicted to methamphetamines and was jailed twice for offences linked to his drug use.

KARL O'CALLAGHAN: He says it was cannabis that kicked off the mental health issues and I noticed at the time a change in his attitude, a change in his connection to the family, and a change in his connection to the world generally.

He became apathetic and he lost interest in what was going on around him and I think that's where he lost his way.

PROF. PATRICK MCGORRY, PSYCHIATRIST: Most of the drug use that we see in young people is actually secondary to distress and mental health symptoms and people are self-medicating.

If people are helped, if young people, in particular, are helped to resolve their mental health problems in a more effective way, then that would definitely be a way of reducing the harm associated with drug use.

GEOFF THOMPSON: Former Australian Of The Year, Professor Patrick McGorry agrees that cannabis use should be treated as a health issue - though he's in two minds about whether a legal marijuana market is the answer.

PATRICK MCGORRY: I don't think we should legalise it immediately.

I don't think we're clear enough about the relative balance between the risk and benefits but I think we should definitely strongly support research, measuring those risks and benefits.

There are definitely pros and cons and we ought to think about it pretty carefully.

GEOFF THOMPSON: Mick Palmer says young police officers often have much in common with the people they are arresting.

MICK PALMER: Many of those have probably recreationally from time to time used drugs themselves either before or even while they are serving police officers.

It puts those young police officers at conflict with and forms a distrust between themselves and the young people they have to deal with, charge and arrest.

When, in fact, we should be creating an environment of trust and cooperation.

KARL O'CALLAGHAN: There are a number of laws that are very difficult to police. But the fact that the law exists actually does send a message to the community that it's important.

RICHARD DI NATALE: Nearly seven million Australians have done it.

The people choose to purchase cannabis and if they are going do do that, they should be able to purchase a substance of known quality and purity - a lower risk substance.

At the moment, that's all in the hands of criminals and drug dealers and what it means is, it makes the choice that Australians are already making, a much riskier one.