Criminal behaviour is generally seen as fairly black and white: an act is either legal or illegal.

But, when it comes to possessing cannabis, the situation in the ACT will be very hazy from early next year.

And it may take having a very unlucky Canberran arrested, charged and put before the court to clear the (green) air.

There are currently two very different views on the legal status of someone wanting to consume cannabis under the new laws.

The ACT Government thinks such a person would be on solid legal ground and could not be convicted.

Meanwhile the Federal Government warns a conviction remains very possible if the law is properly enforced.

So who is right? Can a cone put you in the clink?

It will likely fall to a magistrate or judge to make that call.

Can cannabis be legal and illegal … at the same time?

The ACT Government passed laws last month that essentially fully decriminalise cannabis under certain and very specific conditions.

When the legislation comes into effect on January 31, 2020, it will be legal, under ACT law, for adults in Canberra to grow, smoke and own small amounts of cannabis.

But the Commonwealth has laws of its own in this area, and those laws explicitly prohibit possessing any quantity of cannabis (with the exception of medicinal cannabis, which is a completely separate thing).

Conflicting briefs: Federal Attorney-General Christian Porter (left) and his ACT counterpart Gordon Ramsay. ( ABC News )

The ACT drafted its legislation with the Commonwealth laws in mind, and it is relying on a provision that allows anyone charged with a Commonwealth offence to "justify or excuse" the conduct because it is legal under state or territory laws.

Basically, the ACT cannabis laws exploit a very clear and deliberate loophole within the Commonwealth laws. Or so the ACT thinks.

Attorney-General Christian Porter made it clear on the weekend he firmly disagrees.

"My advice, and the advice that I've provided to the ACT Attorney-General, is that it is still against the law of the Commonwealth to possess cannabis in the ACT," he told Insiders on Sunday.

He argued that because the ACT laws really just removed any penalties for possessing cannabis, rather than explicitly legalising it, that loophole did not apply.

And he expects ACT Policing, which is a branch of the Australian Federal Police, to enforce the Commonwealth law prohibiting cannabis.

Will you be arrested? Maybe. Convicted? Maybe

Determining who is right or wrong on that one will possibly fall to the courts, and could require at least one unlucky person becoming a closely watched test case.

ACT police are entirely within their rights to arrest and charge someone in Canberra with cannabis possession, and hand the case to Commonwealth prosecutors to take before court.

A magistrate or judge would then decide how the law should be applied and set a precedent for others to follow.

ACT lukewarm on legalising cannabis: A slim majority (54 per cent) of Australians think marijuana should be legalised

A slim majority (54 per cent) of Australians think marijuana should be legalised The ACT has one of the country's lowest proportion of residents in favour (49 per cent) despite its reform agenda

The ACT has one of the country's lowest proportion of residents in favour (49 per cent) despite its reform agenda Tasmania is most supportive with 56 per cent of residents in favour of legalisation Source: Australia Talks National Survey (54,000 respondents)

Professor Desmond Manderson, from the Australian National University's law college, suggested this would be the most likely outcome.

"There will have to be a court case to work out the meaning of the provisions in the Commonwealth Crimes Act that recognise the freedom of state and territory governments to make their own drug laws," he said.

"And, really, the sooner the better."

Federal prosecutors initially advised that Canberra smokers would be protected, then rescinded that advice.

The role of Commonwealth prosecutors is interesting, as the ACT's position is partly based on advice provided by the Commonwealth Director of Public Prosecutions to the ACT before the cannabis bill was passed.

The Commonwealth DPP advised that the legislation would likely be available as a defence to someone charged with a Commonwealth offence, and as such it might decide not to prosecute.

However, it rescinded that advice just a week after it provided it.

Professor Manderson believes the ACT is on reasonably solid ground and cannabis users will be protected by its laws.

In fact he said the Commonwealth loophole even went as far as to protect those who "reasonably believed" they were acting legally under separate state or territory laws.

"It seems to me, if the ACT Government believes, as it obviously does, that the territory law protects ACT consumers from federal prosecution, it would surprise me if that didn't provide the basis for a strong defence by users to Commonwealth prosecution," he said.

Lawyers want more protection for those getting baked

The Law Society sees an obvious way around all of this, without putting an unlucky Canberran through the hardship of arrest and prosecution just to work out which law prevails.

It wants the ACT Government to come to an agreement with ACT Policing not to enforce the Commonwealth law.

The society's Michael Kukulies-Smith said the current situation left both cannabis users and police officers in an untenable position.

"The Law Society is concerned that the potential for police to still lay charges under the criminal code may lead to inconsistent outcomes for Canberrans based upon the attitudes and approaches taken by individual officers," he said.

According to Mr Kukulies-Smith, a formal agreement would resolve the problem.

Failing that, he said, it may come to a head in court.

However, the Canberra Liberals argued that was far too great a risk to place on ordinary cannabis users — if the laws were bungled, they should be scrapped.

Shadow ACT attorney-general Jeremy Hanson said there was still time to put a stop to it all.

"They're going to be using ACT citizens essentially as crash test dummies for their incredibly flawed legislation," he said.

"When you've got advice from the federal Attorney-General that the law is invalid, then it is reckless to proceed."