It was likely a crackdown, particularly over summer growing months, was reflected in the number of arrests being made.
"Wairoa police and also the Eastern District Organised Crime Unit have really put an emphasis on drug dealing whether it be cannabis or methamphetamine in the past four to five years," Mr Foster said.
"It's been a problem that's typical on the East Coast because it has very good growing conditions for cannabis, there's always going to be a lot around, hence the focus on drug dealers and growers."
Rural areas often had a better climate, making them ideal hot spots for marijuana operations.
One of the main reasons for a crackdown across the board was that the trade had a domino effect on other areas of crime.
"Drugs fuel family violence and property crime, normally burglary, if we can target drug dealers both the family violence and burglary figures come down."
Eastern District Police spokeswoman Kris McGehan said cannabis had always been an issue in Hawke's Bay, as it was throughout the country.
The drop in Napier and Hastings convictions reflected a drop in crime overall.
"Since the 2008/2009 fiscal year there has been a 20 per cent drop in recorded criminal offences," she said.
"The reduction in the number of cannabis-related offences in this time period is in line with the reduction in overall recorded crime."
"We undertake annual cannabis recovery operations based on information we receive throughout the year.
"We also closely monitor known drug dealers in the area."
It was too early to assess the effect of last year's synthetic highs ban on cannabis-related offending, Ms McGehan said.
Nationally, district courts convicted more than 25,000 people for marijuana-related offences over the past five financial years.
The total number of convictions 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.
"We've seen in some of the surveys that have been conducted in secondary schools that a whole lot of risk-taking behaviours - cannabis use, tobacco use, alcohol consumption - are coming down, so that seems to be a cultural shift.
"People are getting a whole lot of messages around tobacco - it's expensive, there's age restrictions, there's horrible pictures.
"Young people seem to be picking up an associated message that any kind of smoking isn't healthy."
Mr Bell believed broader education around substance harm and the importance of being healthy would further bring the numbers down. He didn't think changes to police enforcement or the availability of synthetic cannabis would have much of a reduction effect.
