The ACT Government has sent the Commonwealth a strongly worded defence of its cannabis legislation, warning against any attempt to overrule the contentious law and "will of the people".

Key points: A new ACT law that legalises the possession and use of small amounts of marijuana will come into effect next year

A new ACT law that legalises the possession and use of small amounts of marijuana will come into effect next year The ACT's Attorney-General says federal police have better things to do than pursue cannabis users

The ACT's Attorney-General says federal police have better things to do than pursue cannabis users ACT Chief Minister Andrew Barr says he does not expect cannabis use to increase after legalisation

Senior federal ministers have been highly critical of the ACT's move last month to legalise possession, consumption and cultivation of small amounts of cannabis for adults in Canberra from early next year.

Federal Attorney-General Christian Porter is still weighing up a response to the legislation, which his office only saw in full this week.

The ACT Government sent two letters about the laws to the Commonwealth yesterday. The first, from ACT Attorney-General Gordon Ramsay, warned Mr Porter against any challenge to the legislation.

"The Act has been passed by the ACT Legislative Assembly as a self-governing body with the authority to make laws for people of the ACT," Mr Ramsay wrote.

"I reiterate my previous advice that this legislation expresses the will of the ACT people and convey my deep concern should the Commonwealth seek in any way to overrule and intervene in the democratic processes of the ACT."

The Commonwealth has expressed "deep concerns" about cannabis's links to mental illness. ( Sharon McCutcheon: public domain )

The Commonwealth has the power to strike out laws made by the ACT and Northern Territory parliaments. It last did so in 2006, when the Howard government overturned the ACT's decision to recognise marriage-like civil unions between same-sex couples.

The new ACT law directly conflicts with Commonwealth laws banning the possession of cannabis, a matter Mr Porter previously flagged he would examine closely.

However, in his letter, Mr Ramsay argued there were provisions within Commonwealth law that allowed the ACT law to be applied without issue.

He also said federal authorities should spent their time addressing matters more serious than simple cannabis possession.

"I trust that the time and resources of our federal police and courts will not be wasted pursuing individual cannabis users who are acting in accordance with ACT law," Mr Ramsay wrote.

'Adverse effects' don't make law a bad idea: Chief Minister

In a separate letter, Chief Minister Andrew Barr responded to federal Health Minister Greg Hunt, who had demanded to see what evidence the ACT used to justify its law.

Mr Hunt outlined his "deep concerns" about the ACT's decision last week, and sent a dossier of research and briefings on the risks of consuming marijuana.

Mr Barr accepted Mr Hunt's arguments that there were serious physical and mental health risks around using cannabis, but said the important issue was how to best manage those risks.

"The ACT Government does not dispute that cannabis use can have adverse effects on personal health and well-being," Mr Barr wrote.

"It is the Government's view however that the outright prohibition of cannabis is of limited, and often negative, effect when seeking to reduce the harms caused by cannabis use."

Federal government ministers have described legalising marijuana as "crazy" and a "very bad idea". ( ABC News: Niall Lenihan )

Mr Barr made it clear the ACT Government neither condoned nor encouraged cannabis use, and did not expect its use to increase once it was legalised next year.

He said research from Europe indicated no relationship between changing penalties for cannabis possession and the drug's use.

"The health risks you have outlined, and which the Government is aware of, already exist for the 8.4 per cent of Canberrans that have reported using cannabis in the previous 12 months," he wrote.

"The change will assist cannabis users to reduce or stop their use of the drug by reducing stigma and the risk of punishment associated with criminal penalties for possession.

"Removing unnecessary and ineffective penalties creates opportunities to better reach people already using cannabis, to help connect them with the services and supports they need."

The legislation is still expected to take effect from late January next year, and a public information campaign is expected to run before that date.

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