Investment is needed to develop accurate drug-driving tests, says MP Chlöe Swarbrick, but she says "old school" testing of walking in a straight line is currently the best way to test for impairment.

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The Green Party drug reform spokesperson told TVNZ1's Q+A the issue of people driving while impaired by drugs is concerning many New Zealanders ahead of next year referendum on cannabis legalisation.

"That's a massive issue, whether we're talking about cannabis or we're talking about alcohol."

Yesterday, a petition was re-launched urging the Government to introduce random roadside drug testing.

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"We know what's on the table doesn't pick up prescription, doesn't pick up a number of products like synthetics", Ms Swarbrick said, and on cannabis, "there are a lot of false positives".

"There's a lot of testing going on at the moment to improve those regimes. We currently don’t have the perfect technological solution. What we do have is old school impairment testing," she said.

"Getting people, whether they are drunk or they are drugged walking a straight line, touching their nose…. No it’s not sophisticated but it’s the best way to test for impairment."

Yesterday, deputy Prime Minister Winston Peters was asked the Government's stance on introducing drug-driving tests.

"What we've got to make sure we've got first is the scientific utilities all over the country to ensure when you go for the test it is scientifically accurate, and we haven't got those utilities now."

He said the discussion document on road side saliva testing will go out to public for consultation "very, very soon".

Mr Peters was also asked of the potential of drug-driving tests being introduced prior to the cannabis referendum at the 2020 election.

"You mean, if there is the potential for there to be a 'Yes' vote, it heightens the need for both the law, the men and women, and the scientific equipment to do the job? The answer is profoundly, yes."

On Q+A, Ms Swarbrick also ruled out the possibility of cannabis infused gummy bears or slushies being sold legally.

"We are pushing for legalisation and regulation, because that is the only way to ensure we are getting it out of the hands of kids and gangs. We want to delay the onset of that usage and make sure it is not a glamourised product."

Ms Swarbrick said the proposed 20-year-old age limit for cannabis use was about "finding that line where people can make common sense decisions about their own wellbeing".

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She also said she would like see criminal convictions for cannabis use expunged.

"My personal belief is yes and we are working to negotiate that with our Government partners."