The driver of a 26-wheeler paint truck which overturned on the M1 north freeway in Johannesburg is in a serious condition, paramedics said on Monday.

Netcare 911 spokesperson Nick Dollman said the man's left leg had been "traumatically amputated" above the knee when the truck overturned.

"Paramedics and fire fighters had their hands full trying to get the trapped driver out of the wreck. Jaws of life were used to pry a small opening between the roof of the truck and the road surface," he said.

The driver was taken to a trauma unit at Milpark Hospital where he was placed onto life support machinery shortly after arrival.

The Johannesburg metro police earlier reported that the truck which overturned early on Monday morning had left about 200 metres of the freeway covered in white paint.

Superintendent Wayne Minnaar revised this to 400 metres of the road surface.

"The wreck has been moved but the M1 north freeway is still closed while cleaners try to remove the paint. Sand blasting machines with water are being used to clean the road.

"Two lanes have since been created on the M1 south for traffic to contra-flow," he said.

Motorists were advised to be patient and not to make u-turns on the freeway but rather avoid the M1 north route and use the M2 east.