Northland has one of the highest rates of drugged drivers in the country, but 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, including 17 in Northland. 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.

Of the non-metropolitan areas, only Bay of Plenty and Central had more drugged drivers caught, 30 and 22 respectively, than Northland, but those areas had larger populations.

Northland is also recognised as the cannabis capital of the country, with more dope plants found by police in the region every year than any other.

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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, so the figures didn't show all the cases where drugs were suspected.

NZ 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.

"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, Mr Bell said.

The New Zealand Transport Agency had television commercials about cannabis and driving, including a drug-affected driver buying 12 frosty pigs from a bakery and three children playing in an old car talking about their parents driving stoned.

Northland acting road policing manager Inspector Wayne Ewers said driving under the influence of drugs was certainly a problem in Northland. Mr Ewers said police were educating the public on the dangers of drug-impaired driving as well as having officers out on the roads enforcing the law.