Drivers testing positive for drugs they have not taken could prove problematic for any plans to adopt a roadside drug driving testing scheme in New Zealand.

But experts and advocates say the policy is still worth pursuing to help save lives on New Zealand's roads.

This week, Associate Transport Minister Julie Anne Genter and Minister of Police Stuart Nash called for public consultation on the issue of drug driver testing.

The latest data from the Ministry of Transport shows that drugs were the reason for 21 per cent of fatal crashes in New Zealand in 2017 – ahead of alcohol at 18 per cent.

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If adopted, New Zealand would not be the first country to have such a system – the UK, Canada and Australia all have similar regimes.

Despite this, implementing roadside drug driving tests in New Zealand is not as simple as it may seem.

Genter said as much on Wednesday.

"Unlike with alcohol testing, drug testing comes with some unique challenges, which is why we want expert and public input into the design process," she said.

Unlike alcohol breath tests, drug tests can only detect the presence of drugs or medication. They cannot test if a driver is impaired.

And this is by no means the only problem.

According to a discussion document, prepared by the Ministry of Transport, on enhanced drug-impaired driver testing, the issue of false positives could be a real concern.

"The technology of oral fluid drug detection devices is improving, however, there is a residual risk of screening devices producing false positives," the document said.

It said that in a 2017 study in Canada found that in 7 per cent of cases, where subjects had not used any of the substances, the tests produced a false positive.

In other words, the test was wrong almost one in 10 times.

To put that in perspective in a New Zealand context, in 2017/18 the police carried out 1.7 million breath alcohol tests.

If 7 per cent of those produced false positives, just under 120,000 people would have incorrectly told they were over the limit.

Genter was asked about this issue after releasing the discussion document this week – she said drug testing was an "evolving technology".

"That's one of the reasons we have taken a careful and robust approach and why we're going out to consult now, instead of just knee-jerk rolling them out.

"We want to make sure that we're going to get a fair outcome and that it's going to reduce impaired driving."

Nash said the Government "was not operating in isolation" and was looking around the world to see what best practice was.

"We can look to implement that in New Zealand."

The issue of false positives in drug testing has come up in Australia, too.

In 2015, a police officer told a motorist, Steve Hunt, he had tested positive for methamphetamine despite never having taken the drug in his life.

Hunt's lawyers demanded the sample be retested, according to the Sydney Morning Herald. When the test came back, it had a negative result and the court case was dismissed.

That was not an isolated incident.

National MP Nick Smith said there was a simple way to mitigate the issue – do the test twice.

He does not buy into many of the arguments against roadside drug testing and has been critical of the Government for taking too long in implementing the test up.

"All these arguments were had when the Government introduced random alcohol roadside testing.

He said the change in social attitude, driven by random roadside alcohol testing, meant the number of fatalities had dropped dramatically.

He said there was the same potential to reduce the number of fatalities from the increasing problem of drug driving, and that was why the Government needed to get on with it.

Helen Poulsen, a forensic toxicologist with the Institute of Environmental Science and Research, said there was no real way to mitigate against the false positive issue.

And there was another issue.

"All those oral fluid tests do is look for the possible presence of a drug – usually cannabis, meth or MDMA," she told the Herald.

"Just because someone has some drug in their oral fluid or even their blood, it doesn't mean that they are impaired by that drug."

However, she agreed with Smith that roadside testing was a good idea as it would help change attitudes around people taking drugs and getting behind the wheel.

"You can't stop everyone, but it's just trying to cut [the number of drug drivers] down.

"If roadside testing, and having that presence of the police with the big booze bus, with drug testing on the side, if that deters some people and prevents more accidents, it can only be good."

- additional reporting by Chris Knox