Young Hawke's Bay offenders often have cannabis addiction problems, an Eastern police youth aid officer says.
Youth Sergeant Ross Stewart said there were two drugs creating major problems for young people in the Bay area: alcohol and cannabis.
"What we are finding is young people who are coming to the police for serious offending or volume offending have alcohol and/or cannabis addiction," he said. The comments follow the release of new research linking ongoing cannabis use before age 18 to a decline in intelligence.
Mr Stewart said there was a particular pattern around cannabis use.
"For kids who continue coming into the youth justice system ... the use of cannabis has a significant effect on youth offending.
"We bring it to the attention of the courts and to Child, Youth and Family who we work with."
Two youth aid police officers worked with schools to raise awareness of issues around cannabis and the harm it could do to young minds.
The report, published in the Proceedings of the National Academy of Sciences, analysed more than 1000 New Zealanders.
It found those who started smoking cannabis during adolescence, and were diagnosed as being addicted by age 38, experienced an average decline in IQ of eight points from the age of 13.
Those who began smoking after age 18, and after their brains had developed fully, did not show a similar decline.
The Secondary Principals Association says cannabis use among young people and its effect on learning is an increasing concern for schools.
"Since the introduction of zero tolerance for drivers [under 20], who are not allowed to drink any alcohol at all, we've noticed that some students ... or teenagers are switching to the use of cannabis because that's not detected," association president Patrick Walsh said.
"We have reports where students take up the marijuana habit at the age of 12 or 13. And the students who are regular users invariably come from homes where there is use of it."
Schools were struggling to address the problem of cannabis use by students, who often experienced short-term memory loss and lacked motivation once they started using. Some schools already carry out random sniffer dog testing for drugs.
But Mr Walsh said if young people were switching to cannabis because it could not be detected during roadside breath testing , "then I think drug testing needs to be seriously considered".
Social Development Minister Paula Bennett announced compulsory drug testing for beneficiaries earlier this week. Under the scheme, which will take effect next July, beneficiaries who fail or refuse drug testing when applying for jobs will have their welfare payments stopped. They will also have to reimburse the cost of the drug test.
According to the United Nations, Kiwis have the highest rates of cannabis use in the world.