The Ministry of Transport downplayed the prevalence of drug driving in a paper that went to Cabinet this year.
Radio New Zealand's transport reporter says a large section of information in a draft paper was deleted from the final version.
Among the deleted information was work the ministry commissioned to analyse the blood samples of 453 people taken to hospital after a crash for which they were deemed at fault. Many had been using drugs.
The ministry also says recreational usage of drugs in the population is pervasive.
The draft version of the Cabinet paper had 2½ pages dedicated to the size and nature of the drug driving problem in New Zealand. There was half a page in the final version.
The ministry says standard editing processes are used when writing technical papers to make them more readable.
The group Campaign Against Drugs on Roads says such information should have been included for ministers to see.
AA raises breathalyser concerns
The Automobile Association is concerned that drivers who combine drugs and alcohol could be passing roadside breathalyser tests.
The association wants the government to adopt random saliva testing to detect combination users.
Spokesperson Mike Noon says combination drivers are impaired after mixing small amounts of alcohol with drugs.
He says an Environmental and Scientific Research report found drugs, or a combination of drugs and alcohol, in the systems of more than a third of 1046 drivers.