Mr Megaw admitted the drugged driving impairment tests were not particularly common "as evidenced by the fact that we haven't actually prosecuted anyone for it".
But he said it would be naive to think that no one drove while solely under the influence of drugs.
"Unfortunately we haven't apprehended them."
Drugged-drivers had evaded Wairarapa police so far by driving normally and not exhibiting signs of drug use, he said.
When officers pulled over a car that smelled of cannabis, they then searched the car under the Misuse of Drugs Act, but it was up to their discretion whether to also test them for drug induced impairment.
"We're certainly picking up a substantial number of drink-drivers, there are people who probably are getting through who are impaired but luckily for them they are driving well enough that they're not coming to our attention. They're not weaving or they're not hitting power poles or crashing into other cars, and so they are getting away with it."
AA spokesman Dylan Thomsen said drugged-driving was the "elephant in the room" when it came to road safety.
"This is under-recognised as an issue on the roads.
"We have a huge focus on drink driving, and rightly so, but the evidence indicates that we have an equally large problem with people driving under the influence of drugs and we're not doing enough right now to target those people and keep them off the roads."
The AA wanted to see more random drug testing with saliva testing machines, which are used in every state in Australia.
However, the Government is waiting for saliva testing technology to improve before introducing it.
A government review in May concluded the testing devices were not reliable or fast enough to be effective. Saliva screening takes at least five minutes, is unlikely to detect half of cannabis users, and results are not reliable enough for criminal prosecution, the review found.
A recent Ministry of Transport study found more than half of the drivers taken to hospital after causing a crash had drugs in their system.
Prescription and illicit drugs were detected in 258 drivers, from 453 samples taken.
Currently police can only drug-test drivers if they believe they are under the influence.
The impairment test entails the driver being taken back to the closest police station where they undertake an eye assessment, followed by a walk and turn, and one-leg stand test. A driver who fails the test must undergo a blood test.
National road policing manager Superintendent Carey Griffiths said the current drug testing regime was "very much a work in progress".
"The New Zealand police does not have a quick and accurate test for drugs that can be conducted at the roadside as there is with alcohol.
"However, the intention of the [drug driving ] legislation is to remove impaired drivers from the roads, and therefore, if a driver is found to be under the influence of alcohol, police do not usually take this further and test for drugs also.
"The vast majority of impaired drivers are impaired by alcohol and at this stage we do not have accurate information as to the extent of drug driving in New Zealand."
Facts
Nationally, 721 people have been charged with driving under the influence of drugs since November 2009 - 583 under the new legislation and 138 under the old.
No one in Wairarapa had been charged by the end of August.
