One drug detection expert says that one of our ministers is confused on the science of drug testing.

The Government yesterday launched public consultation on roadside drug testing, but the family of one man killed by a drugged driver says it is a waste of time.

Peter Dow, whose son Matthew was killed by a driver high on meth, says that the Australian state of Victoria has been doing this for 15 years and we should follow their lead.

Associate Transport Minister Julie-Anne Genter, however, told Mike Hosking there are issues to be looked at.

"The difficulty is in getting a technology that can actually establish impairment that can test the most common substances, and can do it at a scale like alcohol breath testing."

However, Kirk Hardy, CEO of the Drug Detection Agency, told Heather du Plessis-Allan that drug testing can test how much of a drug is in a system, but people get too focused on impairment. 

"Toxicology cannot test for impairment. It just cannot happen. I think Julie-Anne Genter is a little confused on that note there." 

Hardy says that currently New Zealand has compulsory impairment testing officers in a similar manner to the United Kingdom. These officers put drivers believed to be under the influence through a series of test such as an eye-test and a 'walk and turn'.

"That's a good test for impairment, and then if you fail that test, you will go on to see a medicinal practitioner and you will do a blood test." 

He says that in Australia, there test for mere presence of a drug in saliva, and that presence is what people are charged on. 

Hardy says that legislation needs to set a clear definition on what is considered under the limit, as how drugs and alcohol affect people does vary between person to person.