Police have no idea how big the drug-driving problem is in New Zealand.
Only two people have been prosecuted for driving under the influence of drugs since it became an offence on 1 November, 2009.
New legislation allows the police to test drivers for impairment and to test their blood for drugs if they fail.
Superintendent Paula Rose says alcohol is a well known influence on driving, but there is little research on people who are behind the wheel after using cannabis or amphetamines.
She says it will take some time before police know how big a problem they are dealing with and how to keep up with the changing drugs being used.
However, an anti drug-driving group, the Candor Trust, says ESR blood tests completed for the police on crash victims since 2004 show 16% had drugs in their system at the time.