Wairarapa police believe they are only "scratching the surface" of rural cannabis operations when they do aerial spotting.
Fewer people are being convicted on cannabis charges in Wairarapa, but Detective Sergeant Barry Bysouth says police believe the same amount is being grown now as five years ago.
Ministry of Justice figures show that the number of cannabis-related convictions in the district court dropped from 85, five years ago, to 54 in the last year. The convictions included dealing, trafficking, manufacturing, cultivating, possessing, using and other illicit drug offences.
Possession and use convictions had the biggest decrease - from 39, in 2010, to 21 last year. Dealing and trafficking convictions stayed relatively steady over the five-year period, while manufacturing and cultivating convictions fluctuated between 15 and 29 a year.
There were no importing or exporting offences.
Mr Bysouth says police know that cannabis is being grown and used fairly frequently throughout Wairarapa.
"We do what we can to prevent ongoing abuse of substances but, ultimately, the Wairarapa, in particular, and New Zealand, as whole, has such an easy climate to grow cannabis in.
"We probably only scratch the surface of it when we fly to rural areas to spot cannabis."
Greater use of formal and pre-charge warnings by police, for possession of small amounts of cannabis and utensils, may account for some of the reduction in convictions, he says.
"Police take investigations into cultivation, supply and possession of large amounts seriously. Those type of offences won't receive warnings."
Mr Bysouth said he'd like to think society, as a whole, was becoming more aware of the social harm cannabis created. [Police] frequently go to schools and other agencies to talk about the effects of drugs and alcohol.
He said it was difficult to say whether last year's ban on synthetic highs had affected cannabis use.
"We're aware there's still some synthetic drugs being used in the region but it's hard to say if those users are also using cannabis."
Everyone had a responsibility to reduce the harm caused by cannabis, he said.
"If you have a friend or family member affected by substances, don't turn a blind eye to it. Police aren't just here to enforce the law.
"We also have a social responsibility.
"You can contact us anonymously and we can help you through the issue or you can seek help from a number of different agencies."
Nationally, district courts convicted more than 25,000 people for marijuana-related offences during the past five financial years.
The number of convictions has decreased each year " from 7329 in 2010 to 3480 last financial year.
New Zealand Drug Foundation executive director Ross Bell believed greater use of pre-charge warnings by police was the main reason for the decline.
"The simple explanation is police are processing people differently," he said. "I think there's a small decrease in the number of people using cannabis but not big enough to account for those numbers."
Mr Bell expected greater education around the harms of substances had turned some people off the drug.
"Young people seem to be picking up an associated message that any kind of smoking isn't healthy."