The New Zealand Drug Foundation boss says new technology is needed to give a clearer picture of the drug-impaired driving situation in New Zealand.
Police recorded 205 drug-impaired driving prosecutions last year to the end of September. There were 209 recorded for all of 2014 and 250 in 2013.
Police said a single incident could generate a number of prosecutions for one person.
The figures didn't show all the cases where drugs were suspected. When both alcohol and drugs were suspected, an officer would usually test for alcohol first as it was easier to get a conviction and the road safety outcomes were similar.
New Zealand Drug Foundation executive director Ross Bell agreed it was easier to test for alcohol than drugs - and road safety aims were achieved by getting drugged drivers off the road who were also impaired by alcohol.
"What that means, though, is that we might be missing the big picture around drug driving."
He said some drug impaired drivers may be "getting through the cracks" such as those who were below the alcohol limit but impaired by cannabis.
The situation wouldn't improve until there was technology to test for a range of drugs and measure impairment.
Mr Bell said some countries, including Australia, had moved to saliva testing but that was an imperfect solution.
"All countries are in the same position as New Zealand, that they want to do something about drug driving because it is an absolute road safety problem but enforcement is really difficult because we don't have the technology. So whoever develops the technology is going to have a worldwide market for their devices."
Until that sort of technology was available, more needed to be done around raising awareness of the problem of drug impaired driving, said Mr Bell.
The Drug Foundation had a campaign targeted at young drivers though social media and radio which aimed to challenge myths around the safety of smoking cannabis and driving.
The New Zealand Transport Agency also had television commercials about cannabis and driving, including an affected and indecisive driver buying 12 frosty pig muffins from a bakery.
Mr Bell said he supported more money being spent on public awareness but he didn't think enough police officers were trained to conduct field impairment test and the tests took time.
"That's where the technology would come in if we had it."
The fact a lot of regions were only prosecuting a handful of people, showed police weren't doing a large number of tests, he said.
Bay of Plenty police operations support manager Inspector Kevin Taylor said the number of people caught driving under the influence of drugs was relatively small considering the number of people caught drink-driving.
However, the enforcement provisions around drink driving meant police were better able to intercept and remove drink drivers from the road.
Mr Taylor said Bay of Plenty police had trained more frontline officers in drug-impaired driving detection than most other police districts.
"We take any impaired driving seriously and act accordingly," he said.
Eastern road policing manager Inspector Matt Broderick said there were not a large number of drug driving offences in Hawke's Bay, however some drivers caught over the alcohol limit might have used drugs as well.
"So we may catch someone drink-driving and they may be suffering the affects of taking drugs at the same time. This is not unusual."
Drink-driving was the police's priority in Hawke's Bay, as it was the more prevalent issue, he said.
"We still have a culture in the region where people think it is okay to drink and drive - especially in rural areas - so we spend most of our time trying to prevent road deaths and harm from crashes associated with alcohol."
Mr Broderick said drug-driving offending might not be as prevalent in Hawke's Bay as in other areas, but it was still included in police operational plans.