The government has told police to catch more drunk drivers and stop pushing to saliva test drivers for drugs.

Documents obtained under the Official Information Act show the Transport Ministry and police want 45,000 motorists a year to be randomly tested for drugs at a cost of $9 million.

The tests would detect cannabis, meth and ecstasy.

Each test would cost about $45 to administer and drivers would have to wait at the checkpoint for five minutes until the result came in.

The Transport Ministry argues it's justified in order to deter motorists from driving on drugs, and estimates it would lead to six fewer fatalities a year.

The report obtained by RNZ was originally given to the previous National government which repeatedly declined to introduce saliva testing.

It was also given to the new Minister responsible for road safety Julie-Anne Genter.

Julie-Anne says the Police should have other priorities.

"What we've seen in the last few years is there's been a reduction in police resource put into dedicated enforcement of road safety - including alcohol testing.

"So given alcohol testing is so much cheaper and there's a much greater number of people who are impaired on alcohol driving on our roads I'd like to see that increased, before we start saying we're going to spend lots of money on saliva testing."

The number of alcohol breath tests carried out by police has plummeted by more than 40 percent in the last five years - while the road toll has risen by 50 percent.

The Transport Ministry says in Australia for every 50 saliva drug tests carried out - one driver returns a positive result, although sometimes those are false positives.

In February, Police Commissioner Mike Bush said police were heading to Australia to observe the drug testing of motorists there to see if there is a better way of doing things.

But Julie-Anne is not convinced.

"From my perspective it is not clear that saliva testing is the best investment of money to reduce impaired driving and reduce dangerous driving, and reduce serious injuries and deaths."

Drug Foundation executive director Ross Bell says saliva tests did not detect the full rate of drugs that can impair motorists and had major flaws.

"We are really concerned to see in the last few years police law enforcement around alcohol related driving, the roadside random tests, all of that enforcement has dropped significantly - so it doesn't surprise us actually that the road toll is going up.

"So I think we need to get it right around alcohol first."

Leah Abrams founded the support network, No One Ever Stands Alone, for the victims of drunk drivers after she was injured by a drunk motorist in Auckland in 2015.

She supports saliva testing.

"We always need to be getting out and doing more checkpoints to stop drunk drivers.

"But at the same time I think, in our society as drug driving becomes increasingly more of an issue, we need to start planning for the future and start equipping the police with the necessary tools that they need."

Police currently carry out co-ordination tests when they suspect a driver is high on drugs.

Recently, between 300 and 400 drivers fail each year and have blood samples taken and sent for analysis.

More than 90 per cent of motorists who fail the co-ordination test then test positive to a impairing prescription or illegal drug.

The Police refused to make anyone available for an interview to explain what they learned on their trip to Australia.