Roadside drug testing will be rolled out in New Zealand by 2021, the Government has announced.

It means police will be able to test for drug driving at roadside stations, like those used to stop people from drunk driving.

"The new powers will send a clear message that if you take drugs and drive, you will be caught," said Associate Transport Minister Julie Anne Genter.

The law change will mean police will have enhanced powers to conduct drug tests at roadside testing stations.

Drivers who test positive will be fined and stopped from driving for a minimum of 12 hours.

WARWICK SMITH/STUFF Roadside drug testing will be introduced next year.

They could also face criminal penalties if they fail a compulsory impairment test and a blood test confirms "impairing levels" of drugs in their system.

The threshold for "impairing levels" of drugs would be aligned with levels for alcohol.

"This means a blood test that identifies impairing medication or drugs at or above an amount equivalent to the criminal drink driving limit (80mg of alcohol to 100ml of blood) will result in a criminal offence," Genter said.

The AA's road safety spokesman Dylan Thomsen said the move was "fantastic".

"We see this as a milestone moment for New Zealand road safety history," Thomsen said.

Drug driving was "a silent killer on New Zealand roads".

"People don't realise how many crashes have involved people with drugs in their system," Thomsen said.

CHARLOTTE COOK/ RNZ Associate Minister of Transport Julie Anne Genter has announced roadside drug testing would be introduced next year.

​Genter said that last year there were 95 people who were killed in preventable crashes where drugs were involved.

Roadside drug testing is popular in Parliament, with National also calling for it to be introduced.

Genter said the delay had partly been to do with the fact drug testing was more complicated than alcohol testing.

"It is a bit more complex for a variety of reasons: drugs affect people differently, there's a variety of substances, and we have to control for the fact they [saliva tests] aren't always 100 per cent accurate," she said.

The scheme will require new legislation to be reintroduced, which will happen in the New Year. This will allow roadside testing to be up and running next year.

Police will use an oral fluid test to test for drugs. Police will test for THC (cannabis), methamphetamine, opiates, cocaine, MDMA (ecstasy), and benzodiazepines

Some prescription drugs will also be tested for, but Genter said there would be a medical defence for people who were driving with drugs in accordance with their prescription, although just how this worked had not yet been developed.

Genter said roadside drug testing was just one of a series of changes the Government would be making as part of its road safety programme, which will be announced in more detail on Thursday.