Eileen Tamati's husband Kent Martin was killed by a drugged-up driver last year. Photo / Supplied

Eileen Tamati's husband Kent Martin was killed by a drugged-up driver last year. Photo / Supplied

New research suggests New Zealand's drug-driving epidemic may be worse than officials thought.

An online survey of 1200 Kiwi drug users shows one in four drove after taking cannabis in the last year, while 21 per cent admitted drink driving.

The Drug Foundation survey has top officials worried.

As police prepare to enforce new drug-driving tests from December, Transport Minister Steven Joyce said the problem "may be bigger than previously known".

"Police, I think, are keen to get the enforcement under way," he said. "If you're impaired while driving you're then tested for drugs. If you fail that test then you are charged."

Drug Foundation executive director Ross Bell was surprised by the survey results.

More than half of respondents said taking cannabis did not affect their driving ability and more than 16 per cent said it improved it.

"That has real, obvious implications for the road safety messages we need to get out to people who use drugs."

Fourteen per cent of respondents said they had used other drugs, most commonly methamphetamine, methadone and morphine, while driving.

But Bell said the new police tests may not solve the problem.

He described them as "dodgy American-style field impairment tests where they get drivers to walk in a straight line and those kinds of things".

"It just doesn't have the same degree of rigour that we would want in a drug-testing regime."

Environmental Science and Research is working on a long-term study analysing blood samples from most driver deaths in New Zealand.

Preliminary results show 40 per cent of drivers killed between 2004 and 2006 had used alcohol, cannabis or both before their death.

Thirty per cent of drivers who used cannabis and died in a motor vehicle accident were likely to have smoked within three hours of driving.

But, without the technology to detect drug users quickly, the number of people convicted of drug-driving is low.

Provisional police data shows 69 people were charged with driving under the influence of a drug last year, compared with 60 in 2006 and 84 in 2007.

Only one person was charged with causing death while under the influence of a drug.