The proposal, which will come into effect on January 31 next year, avoided political controversies associated with the large-scale cultivation, manufacture and sale of cannabis - preferring a "home-grown" approach to cannabis consumption. Whilst stoners in our nation's capital rejoiced, questions have been raised about how this legal change will relate to Commonwealth laws that criminalise cannabis possession.

Federal Attorney-General Christian Porter has already warned that drug possession remains an offence under Commonwealth law and that he expects federal police to enforce it. His comments were echoed by Home Affairs Minister Peter Dutton, who told 2GB radio: "I think it might be trendy for the ACT government to go down this path, and they'll say they're enlightened and progressive and all the rest of it ... But I think it's dangerous ... Christian Porter is having a look at it at the moment."

As every high school legal studies student knows, generally when a state or territory law conflicts with the Commonwealth - the Commonwealth prevails. But the legal issues here are a bit more complicated than they might first seem.

Australian criminal law has always had an overlap between federal crimes and state or territory offences. For example, if a vehicle was stolen from a military base in Victoria the joy rider could be charged for theft under s131.1 of the Commonwealth Criminal Code 1995 (Cth) or under s74 of Victoria's Crimes Act 1958 (Vic). Generally, this doesn't pose much of an issue as it provides a choice of both charge and prosecutor.

Federal prosecutors are usually quite happy to let state or territory authorities handle matters under local laws. Indeed, diverse approaches to drug enforcement and prosecution across Australia are not uncommon. Research published in May this year by the National Drug and Alcohol Research Centre found significant differences in approaches to drugs across the nation.

For example, South Australia does not criminally charge 98 per cent of people that are detected for use or possession of drugs, whereas in Western Australia it's only around 32 per cent.

However, ACT's cannabis law poses an interesting question: what should the Commonwealth do when cannabis cultivation and possession is a crime federally but is no longer a crime at a local level?

This was the situation in the United States until quite recently, with cannabis legalised in nine states but remaining a serious criminal offence federally. In the early days of US legalisation federal law enforcement would raid and arrest people in cannabis dispensaries even if local laws allowed for sales. Can Australian Federal Police take the same approach?

Technically, yes. There is nothing stopping the AFP from charging people under federal cannabis offences in the ACT even after the reforms come into effect. This is likely to cause some confusion for law enforcement in the ACT, where members of the AFP are seconded to act as local law enforcement. It's unclear whether police will be instructed to proactively enforce federal cannabis laws or not.

But those charged may have an ace up their sleeve! Under s311.1 of the Commonwealth Criminal Code it is a defence to federal charges conducted in a state or territory if "the conduct is justified or excused by or under a law of that State or Territory". Assuming this defence is available, which ACT Chief Minister Andrew Barr is pretty confident that it is, this would allow users to smoke up and rest easy.

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Another potential spanner in the works is the power given under Section 122 of the Australian Constitution allowing the Federal Parliament to override laws of the territories. Whether the Federal Parliament will seek to do this is unclear.

What does all this mean for cannabis legalisation efforts across Australia? Well, the ACT law provides an interesting avenue - particularly for the States - to subvert federal cannabis prohibition.

New Zealand is set for a referendum on cannabis in 2020 and the Victorian Parliament is currently undertaking an Inquiry into Cannabis, showing that these debates aren't going away any time soon.

Although, one imagines most states will take a "wait and see" approach for the time being.

Whilst Canberrans can inhale and breathe a sigh of relief for now, it may take a while till we see through the smoky haze of federalism.