New devices under evaluation could detect the use of illicit drugs up to six weeks before the urine sample was taken, Police admitted yesterday.

And leading defence lawyer Mark Pettingill said such powerful "draconian'' tests -- soon to be carried out at Police stations -- could be a breach of civil liberties and lead to "unfair'' prosecution based on "nonsense''.

He also questioned what protection the law would give someone if they had inadvertently breathed in other smokers' marijuana fumes at a party or concert weeks before they were tested.

But lawyers and Police in general agreed the stiffening of drink or drug driving laws was a positive move since it would act as a deterrent -- and thereby saves lives.

Ch. Insp. Roger Kendall stressed this type of "kink'' was currently being ironed out of Police procedures before they were introduced.

The cheapest testing device -- able to detect illicit drugs in a person's urine for $10 to $15 per use -- could detect drug use several weeks back.

"It can tell if a person has used marijuana up to six weeks previously,'' he said.

It was an "absolute offence'' to use illicit drugs -- marijuana, cocaine, heroin, or opiates -- and under legislation passed in January the offence could be prosecuted.

"But if someone took it six weeks ago and is now driving is he still impaired? There's bound to be legal argument over that.

"And that is exactly why we're being careful and making sure we can walk with this new legislation before we try to run.

"Technically that person could be charged. We want to prosecute not persecute but at the same time we want to deter people from ever using illicit drugs.

"It is an absolute offence to have them in your system but we do want to be fair to the accused.'' He said another problem was whether over-the-counter drugs such as codeine -- a derivative of cocaine -- would give a false reading.

Top criminal lawyer Mr. Pettingill was outraged at the possible new powers and said a "lot more work'' was needed to protect human rights.

"I can already see a number of difficulties that are going to arise.

"Let's say someone has indulged in minor marijuana usage five weeks before hand and they're stopped. It would be absolute nonsense for that person to be charged and prosecuted, particularly for impaired driving.'' He said what was being considered was "draconian but also very, very difficult to legislate and enforce''.

"The idea of dealing with the effect of drink and drugs on driving and upping the punishment is a good thing, but we can't go overboard.

"And it would seem this is beyond the realms of fair prosecution. "It's all right to make the law stricter but you have to regard people's rights, the constitution and basically what is fair.''