Police currently carry out random alcohol breath tests on any driver.
But they test for illegal drugs only if they have good cause to suspect a driver is under the influence. Even then the current method involves only a manual impairment test. If failed, a blood test is carried out.
The testing gap in the road-safety regime has been on the Government's radar for some time but technology limitations means it hasn't been closed. Australia and other countries do carry out roadside stops using oral fluid tests - also called saliva testing - to detect cannabis, methamphetamine and MDMA (ecstasy).
But a previous New Zealand government review of the drug-testing regime in 2012 concluded results from saliva screening were not reliable enough for criminal prosecution, and the test took too long.
Testing technology has since improved in terms of both speed and reliability.
However, the chances of a false-positive reading mean any test would still need to be followed up with another test, such as of blood, to reach an evidential threshold.
And while speed has improved, initial saliva tests still take up to five minutes - much longer than alcohol breathalysers.
The delay means saliva-testing technology would still be used only when police suspected a driver was under the influence of drugs. Random testing could eventually be introduced as the technology allowed.
Labour's police spokesman, Stuart Nash, said not having drug-testing of drivers was a gap in road-safety enforcement.
"As soon as we get the technology in place I think it has got to be rushed out because people who are high behind the wheel, I think, are just as much a danger as those who are drunk.
"If you can save any innocent Kiwi's life by rushing this in, then let's do it." AA spokesman Mike Noon agreed, and said New Zealand had lagged behind countries such as Australia in having roadside testing.
"It would be very naive to think that drug-impaired driving is not an issue on our roads, simply by the evidence we have seen from the coronial samples that show how many people involved in fatalities do have drugs on board.
"We wouldn't be expecting thousands and thousands of tests but we think it's a very good signal that could be sent - we don't want to encourage people to switch from alcohol, that now has a lower limit, to instead use other drugs."
Ross Bell, of the Drug Foundation, said he was unconvinced that the saliva-testing technology had progressed enough to implement in New Zealand. "Having said that, I think it is probably working well in Victoria [Australia] where it has been running the longest." Mr Bell said more police should be trained to carry out field impairment tests, which were more accurate than many would suspect.
And there was opportunity to improve education around drug-driving, which was currently "abysmal", he said.
"All we have now are those ridiculous ads on TV. That is all Government thinks we need. You could do much more sophisticated education and awareness around this."
The Transport Agency recently launched a new round of "thoughts" television adverts, which feature the wandering internal monologue of a stoned driver before he crashes.
