Sophisticated growing techniques have led to super-strength strains of cannabis, with some of the most potent dope coming from Northland, says a senior drugs investigator.
However, a drug foundation spokesman says it is difficult to draw conclusions on whether the increased strength has led to greater problems with cannabis.
A project involving the police and Environmental Science and Research (ESR) found THC levels (the primary intoxicant in cannabis) was now more than four times stronger than it was when ESR last tested in 1996.
Whangarei police Detective Sergeant John Miller, who has 18 years experience dealing with cannabis-related crime in Northland, said samples sent from the region had continually strengthened and was some of the strongest in the country.
Police and ESR used sophisticated hydroponic equipment to complete three cannabis growing cycles, nursing six plants at a time, 18 in total, to maturity. The study revealed the drug was more than four times stronger than it was last tested in 1996.
THC levels varied between 4.35 per cent and 25.3 per cent during the study completed under Ministry of Health licence between 2004 and 2006. When ESR last tested the Class C drug, it found an average THC level of 6 per cent.
In last year's annual cannabis operation, 34,917 cannabis plants were destroyed during the Northland phase over summer, well over a third of the national total of 97,000 plants.
Police believe THC levels have increased significantly in recent years due to criminals using more sophisticated growing methods, helped by the availability of specialised equipment, like that sold at hydroponic growing shops.
Mr Miller said a decade ago the bulk of the cannabis grown was outdoors but with the advent of indoor operations there was much more control over quality.
"With the indoor operations there comes sophistication and growing, to get an optimum product," Mr Miller said.
Different strains of the plant had been cloned, which had gradually increased THC levels. Northland cannabis samples had also been tested by ESR and regularly ranked near the top, he said.
"We are not seeing poor quality cannabis. It's all good quality."
New Zealand Drug Foundation executive director Ross Bell said New Zealand didn't monitor THC levels in cannabis 30 years ago, but there was no denying cannabis was getting stronger.
Cannabis today was stronger than 30 years ago "but we had very low quality cannabis at that stage", Mr Bell said.
"In terms of [higher quality cannabis] leading to more psychosis, that's supposition because there's not been a great deal of cannabis research."
He said stronger cannabis could mean that smokers used less of it.
"Drugs affect people in different ways and cannabis is the most common illegal drug used here, with pretty easy access to it for many people.
"So it's right to be concerned about the health and mental health effects of cannabis, but it's more difficult to say that because there's higher-potency cannabis the health and mental health effects are going to be far greater," Mr Bell said.
Research on cannabis and psychosis was being done in Dunedin and Christchurch and he awaited the results of that research with interest.
If you notice any suspicious behaviour that could be linked to cannabis, contact your local police station or 0800 CRIMESTOPPERS. All information left on Crimestoppers line will be dealt with by a Northland police officer.