A truck driver who allegedly tested positive to methamphetamine has had his vehicle seized in the Adelaide Hills after police made an unusual discovery while inspecting its rear brakes.

The truck was stopped by police about 6:00am on Monday on the South Eastern Freeway as it travelled towards the city at 90 kilometres per hour.

An examination of the truck revealed vice grips clamping off the rear brakes, while "the front brakes were inoperable", police said in a statement.

Police drug tested the driver, a 37-year-old Campbelltown man, and allege he tested positive to methamphetamine.

He has been reported for acts to endanger life, and police have seized the truck for further checks.

The executive officer of the state's Road Transport Association, Steve Shearer, was horrified to hear of the incident.

"It's nothing short of moronic and suicidal. It's ridiculous," he said.

"My understanding is the clamps were actually used not to apply the brakes but to stop the brakes from coming on, clamping the brakes in an off position. It's just insane.

"It makes no sense at all. The only thing anybody could ever possibly think is that if they've got an issue with their brakes and they keep sticking they'll fix that by clamping them off. Gee, that's like saying 'my steering wheel's a bit stiff. I'll chuck it out the window'."

The truck was approaching the city and was stopped on the South Eastern Freeway. ( SA Police )

Mr Shearer said his association has been pushing the State Government for heftier penalties for truck drivers who do the wrong thing.

"We've got very tough laws, chain of responsibility laws, that cover fatigue, speed, the weight or mass of the truck, the load restraint and the size of the truck," he said.

"But there are no tough provisions to do with roadworthiness and we have fought for almost 15 years to get that fixed and it's about to be fixed next year nationally.

"Because of those laws not being in place on roadworthiness, the penalties for this type of thing are relatively low."