An anti-drugs group says a new drug driving law coming into force this weekend will achieve very little.
From Sunday, police will be able to carry out compulsory tests on drivers suspected of driving under the influence of drugs.
But the Campaign Against Drugs On Roads Trust says people will be tested only if they are exhibiting obvious signs of being on drugs.
The trust says this means large numbers of drugged drivers will get through checkpoints.
It says a random testing campaign is needed for drug driving legislation to have any credibility.
Transport Minister Steven Joyce says he feels the approach being taken to tackle drug diving is the best approach.
He told Morning Report that random testing is not the best way to go.
He said there are less drugs that can be tested for with a random drugs test and impairment tests are the best route to go.