New research has shown a dramatic increase in the number of long-term cannabis users being admitted to hospital.

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

Overall, the rate of cannabis use has been dropping. Around 9 per cent of Australians have smoked the drug in the past year, down from 11 per cent in 2004.

But researchers at the University of New South Wales found that people using the drug are using more, and more often.

The study has been published online in the health journal Addiction.

Amanda Roxburgh, senior researcher at the university's National Drug and Alcohol Centre, says the rate of harmful use has dramatically increased particularly among 30 to 49-year-olds.

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

"And we're seeing particularly in 30 to 39-year-olds, the admissions have increased quite a bit, as have the 40 to 49-year-old age group.

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

Ms Roxburgh says hospital admissions are usually for treatment for cannabis dependence, but a small number are for cannabis intoxication or cannabis-induced psychosis.

She says older users are twice as likely to report daily use compared to the 14 to 19-year-old age group.

She says the rate of harmful use among younger people is also worrying.

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

"So they might be smoking 10 or more joints or cones per day and they're doing that more than the older users.

"Potentially, we might see in five or 10 years - that might be borne out in them presenting to hospital."

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

"We've got to really do some more 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 secondmost drug of concern now, behind alcohol.

"We've got older population groups who've grown up - if I can use that word - using cannabis, and [if we could] get the message to them to understand what sort of concerns they could be inflicting on themselves, we'd probably see some significant improvement."