NSW has some of the highest-potency marijuana in the world, the first Australian research to measure the chemical composition of cannabis seizures has found.

Experts fear increasingly high levels of the psychoactive chemical THC in cannabis could be putting users at risk of developing mental health problems and dependency.

Nearly half of the marijuana samples seized in NSW contained such a high level of the psychoactive chemical THC that in some international jurisdictions they would qualify as being a ''hard'' drug, the study, produced by the National Drug and Alcohol Research Centre, study found.

The research also overturns the commonly held belief that marijuana grown naturally outside is far less potent than that grown hydroponically. The study found there was little difference between the two methods.

Potent: Increasingly high levels of THC have been found in NSW cannabis. Credit:AFP

On average the samples were about 15 per cent THC, which is the main chemical in cannabis that produces the ''stoned'' feeling, but is also linked to negative emotional effects such as anxiety.

Drugs found to have the highest level of psychoactive chemicals, particularly THC, were seized in regional areas such as Byron Bay, Lismore and Tweed Heads.