DANGER: A secret police study involving scientists growing crops of cannabis has revealed New Zealand now has super-strength strains of the drug.

A secret police study involving scientists growing crops of cannabis has revealed New Zealand now has super-strength strains of the drug.

In May last year, Sunday News revealed police and Environmental Science and Research (ESR) used sophisticated hydroponic equipment to complete three cannabis growing cycles, nursing six plants at a time, 18 in total, to maturity.

The findings of the study, to be published in Forensic Science International, and released exclusively to Sunday News earlier this week, revealed the drug is now more than four times stronger than it was when ESR last tested it in 1996.

The THC level – the primary intoxicant – varied between 4.35% and 25.3% during the study completed under Ministry of Health licence between 2004 and 2006.

When ESR last tested the Class C drug, they found an average THC level of just 6%.

The THC levels varied considerably, as did yields, during the latest study – due to the growers lack of cannabis-cultivation knowledge, the ESR report read.

"The inexperience of the growers was evidenced by different problems encountered in each of the three cycles, each of which would be expected to negatively impact the yield and THC data obtained."

Cannabis potency is believed to have remained stagnant from 1976 to 1996. But police believe it has 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 specialist shops some of which police raided across the country this week.

More than 250 people were arrested on more than 750 charges as part of the two-year undercover operation which targeted shops such as Switched on Gardener and large-scale cannabis-growing enterprises allegedly linked to the business and other similar companies.

According to the cannabis study, the first six plants grown were "purchased from an illegal grower" and were a variety known as Red Devil. The second cycle used cuttings from the first, and the last used plants from a police raid – meaning the cannabis grown throughout the study was the same as that commonly purchased by users on the streets.

Scientists yielded more than 5kg of cannabis from the 18 plants. Sold by the ounce, at the going rate of $350, the crop would have been worth more than $60,000.

Scientists grew the drugs in a room "comparable to the average New Zealand bedroom, which is a space commonly used to grow cannabis indoors", the report reads.

"It's a serious drug. And it's very clear that long-term usage has very long-term effects ... it's not the social drug of the 60s any more ... it is a drug that causes serious harm," Detective Inspector Stuart Mills said.

Mills, chief of the National Drug Intelligence Bureau, said the study findings backed-up police fears the drug had become more potent.

"We've been aware through various techniques and our information that THC level has been increasing, and this study confirms it."

Assistant commissioner Gavin Jones said police and ESR undertook the study to determine the size of potential harvests and gauge THC levels which had increased dramatically worldwide over recent years.

Police needed to know how strong our cannabis was so they could "start benchmarking trends and patterns", and compare them to international levels, he said.

"Internationally it's up around the 20% mark, so we're just not quite sure what it is here," Jones told Sunday News in May last year.

The study said the cannabis information was needed to assist in the court process, "when they are considering the severity of the offending, and in particular how much income is being derived from such an illegal operation".

A preliminary investigation into outdoor growing cannabis had shown THC levels averaged almost 11%.