The National Drug and Alcohol Research Centre has found the number of Australians seeking treatment for cannabis-related problems has increased 30 per cent since 2002, with the increase in the rate of harmful use mostly among people over 30.

The rate of cannabis use has been decreasing, with nearly 9 per cent Australians have smoked the drug last year, down from 11 per cent in 2004.

However, researchers at the University of New South Wales found that drug usage among Australians are more and more often.

Among 30 to 49-year-olds, the rate of harmful use is alarmingly increasing, says Amanda Roxburgh, senior researcher at the University's National Drug and Alcohol Centre.

We're seeing an increase in cannabis-related harms, particularly hospital admissions, for things like cannabis dependence in particular, she said.

The admissions of 30 to 39-year-olds and 40 to 49-year-olds have been increasing quite a bit, said Ms Roxburgh.

What we think that reflects is a long-term use over a long period of time.

A small number of the hospital admissions also include for cannabis intoxication or cannabis-induced psychosis, she said.

In comparison with 14 to 19-year-old group, it was noted that older users are twice as likely to report daily use. The rate of harmful use among younger people is also worrying.

She said, When you look at the frequency of use for that group, they're actually much more likely to report the really, really heavy periods of use.

So they might be smoking 10 or more joints or cones per day and they're doing that more that the older users. Potentially, we might see in five or ten years - that might be borne out in them presenting to hospital.

The chief executive of the Alcohol and Other Drugs Council of Australia, David Templeman says the study suggests older users of cannabis are still failing to understand the effects of long-term use.

We've got to really do some work in terms of getting people to understand the impact of cannabis on people, particularly people who might have a predilection to mental health illness and the like, he said.

We now know so much in relation to it. It's the second most drug of concern now, behind alcohol.

The results of the study are published online in the health journal Addiction.