"We know a lot of people switched to synthetics because they were more readily available," he said.
"Cannabis stays in your system for a long time so we also know that people have been making the switch to short-acting substances so they're able to pass Work and Income's drug tests."
Dr Rawlinson said he'd like to think people were also becoming more aware of the harms of cannabis.
Whanganui area commander Inspector Steve Mastrovich said police continued to be vigilant for cannabis crime.
"Every police officer plays a role in reducing drug crime, from early intervention with our officers in schools, to major operations targeting organised criminal networks," he said.
"The annual cannabis and crime operation dismantles criminal networks and removes drugs off our streets and out of our communities."
The public's help was needed to reduce the harmful effects of cannabis, he said.
Nationally, district courts convicted more than 25,000 people for marijuana-related offences over the past five 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 also 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.
"Young people seem to be picking up an associated message that any kind of smoking isn't healthy."
