The son of an elderly woman killed in a horrific head-on car smash caused by a drugged driver has praised the Government for bringing in new roadside drug testing laws.

Last night it was announced police would get new powers to conduct random roadside drug testing, and to prosecute drugged drivers in a bid to save lives on the road.

Those powers will come too late to save Ora Keene, 84, one of seven people killed in the crash near Waverley on June 27 2018.

Her brother Ian Porteous, 80, his wife Rosalie, 76, and friend Brenda Williams, 79, who were with her in the car, also died.

The driver of the other car, Jeremy Thompson, 28, and his five-month-old daughter Shady-Jade Thompson were killed, while Shady's sister Nivek Madams died hours later in hospital. The girls' mother Ani Nohi Nohi was the only survivor of the crash.

An inquest heard Thompson had repeatedly smoked synthetic cannabis before driving that day.

Ora's son Murray Keene was part of a group that has since begged for a law change so police can randomly test drivers' saliva for drugs.

Together with Karen Dow - the mother of Mathew Dow, 23, who was killed on New Year's Eve 2017 by a driver high on meth and cannabis - the families took their fight to Parliament and a select committee hearing where they outlined their case.

Keene was emotional last night on hearing the Government had listened - and believed it was possible the new police powers could have stopped Thompson from driving that day.

"But a lot of things could have happened [differently] from that day. You can't turn back time, that's for sure."

 Thompson were both killed in the crash. Photo / Supplied

The new rules will go through Parliament next year and are expected to come into force in early 2021.

The police will be able to conduct oral fluid drug testing on drivers. Any who test positive will be fined and immediately suspended from driving for a minimum 12 hours.

Drivers will also face criminal penalties if they fail a compulsory impairment test and blood tests confirm impairing levels of drugs in their system.

Keene - who had also lost his aunt and uncle, Ian and Rosalie Porteous, in the crash - said all the work the families had done had paid off.

"Something good came out of tragedy, but it should have happened years ago."

The family had adjusted to the loss of their three relatives and were getting on with life.

"But it takes a while. Like any tragedy you talk about it a lot."

They had always believed the law would change as there was clearly support from first responders and the public.

"The stats just stuck out - like that there are more people dying from drug drivers than drunk drivers. Some people might say it costs too much but what's the cost of a life? $25 for a test is nothing."

Associate Transport Minister Julie Anne Genter said the new powers would send a clear message: "If you take drugs and drive, you will be caught."

Last year, 95 people were killed in preventable crashes where the driver was found to have drugs in their system.

"The change will allow police to test drivers for the presence of drugs and impairing medication anywhere, anytime, just as they can for alcohol," Genter said.

The Drug Foundation's executive director Ross Bell welcomed the Government's move.

He said it represents a change in heart for not just the Government, but also for New Zealand.

"We know drug driving is an issue and we know police need to be equipped with the right tools.

"But the Government has been a bit nervous about rolling out [these tests] because they are not without their issues."

There had been issues in the past with tests producing a false positive reading but it was likely that would be mitigated by technology in the future, Bell said.

But he said it was likely technology would develop in the future which would help mitigate this issue.

Genter, however, said the Government has a plan to tackle the false positive issue.

"The way we have designed this means it does have some practical and pragmatic fail-safes to ensure that we're not getting false positives."

That means if someone fails one test, they will be given another one – if they fail that, "you will face penalties".

The new rules will comes as welcome news to the National Party, which have been campaigning for such a test for much of this year.

Leader Simon Bridges has complained that it has taken so long for the Government to move on this issue.

"National wants a proper roadside drug testing regime in place as soon as possible and before any of the Government's law changes liberalising access to drugs takes effect," he said in May.

But Genter said the legislation had not been delayed – "we have progressed this as fast as we possibly could [have]".

She said she would be surprised if National did not support this when it comes before the House.