Just over 400 compulsory roadside impairment tests were carried out in the first 18 months after drug driving laws came into force - a number road safety advocates say is too low.
From November 2009 to the end of April this year, 417 roadside impairment tests were done on drivers thought to be under the influence of drugs, figures obtained by Radio New Zealand News show.
Almost 300 driving charges were laid and about 200 convictions secured.
A document approved by the Transport Minister in consultation with the Police Minister reveals there was an expectation there would be between 600 and 1000 drug driving offences this financial year.
Automobile Association spokesperson Mike Noon says it appears only those who are completely wasted are made to have an impairment test, and the rest are slipping through the net.
Campaign Against Drugs on Roads co-ordinator Rachel Ford, says the legislation is not being properly utilised.
However Transport Minister Steven Joyce says while the numbers may look smaller than expected, the law is working as intended.
He says police are looking first at alcohol, and a drug impairment test may then be used if a driver passes an alcohol test.
Mr Joyce says those who are under the influence of both alcohol and drugs are likely to be picked up under alcohol legislation, which he says is more proven and is a system that moves more quickly.
Transport Ministry figures show 129 people died in crashes in the past six years where drugs other than alcohol were identified as a contributing factor.
An ESR study conducted between 2004 and 2009 found that 240 of about 1000 deceased drivers used more than one potentially impairing drug.
