More than 490 people were arrested for illicit drug offences in the ACT, a rate of 117 per 100,000 people.

Nationally the rate was 609 per 100,000, according to the Australian Criminal Intelligence Commission's report on illicit drugs covering the 2018-19 year.

Almost half of the drug arrests in Canberra were for cannabis, three-quarters of which were consumers not dealers, some of whom were issued a warning.

In ACT, total drug arrests fell 23 per cent over the five years to June 2019, compared with a national reduction of just 1 per cent.

Analysis from the Alcohol Tobacco and Other Drug Association ACT found the reduction was driven largely by arrests for cannabis, with ACT arrests down 43 per cent, compared with a fall of 11 per cent nationally.

Much of the reduction in the ACT was in the last reporting year. Arrests from 2017-18 to 2018-19 for all drugs fell 34 per cent in the ACT and 38 per cent of cannabis.

Cannabis was not decriminalised in Canberra until July 1, after the data was collected.

The Australian National University School of Criminology's Gabriel Wong said the trend reflected police discretion regarding the enforcement of Cannabis offences, as a response to increased public approval.

Dr Wong said the expectation following decriminalisation was that there would be an increase in the proportion of cannabis arrests for providing cannabis compared to consumption, although not of commercial quantities.

"It's likely that, simply, more people will be growing plants in their households," he said.

Ignorance of the law would likely see people engaged in exchanging cannabis with associates arrested for dealing, Dr Wong said.

According to the National Wastewater Drug Monitoring Program, the consumption of cannabis decreased in both capital city and regional sites during the last reporting period.

Dr Wong said while consumption might be going down, the 2019 National Drug Strategy Household Survey found approval for cannabis was going up.

"We expect decriminalisation to further that trend with more Australians approving the use of cannabis," he said.

"This was also the first year we saw cannabis have higher level of approval than tobacco, which is very interesting."

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ACT Drug Association chief executive officer Devin Bowles said he commended any focus of police resources away from enforcement of minor drug offenses toward serious crimes.

"The evidence is clear that issues associated with personal drug use are best addressed through the health system, not the justice system," he said.

Mr Bowles said in the year ending June 2019, more than one in five drug arrest in the ACT were providers, compared to one in 10 nationally.

"This suggests that police in the ACT are doing a better job than the national average of focusing their resources on drug providers rather than drug users, but there is still a long way to go to ensure that people aren't being arrested for personal use offenses," Mr Bowles said.

"Canberra has historically been more progressive than many other parts of Australia on drug policy.

"We have been fortunate to have great leadership in the drug policy space from some thoughtful and courageous people.

"This includes the grassroots campaigners who pioneered publicly available naloxone to save lives, and politicians who have progressively rationalised drug laws.

"We hope this work will continue."