Man crushed under his own truck in Sydney

A man is in a critical condition after his truck rolled over his legs when he got out to inspect its brakes on Sydney's M1 Pacific Motorway.

Emergency services were called to the motorway at Wahroonga just before 5am on Tuesday to treat the truck driver, aged in his 60s, for critical injuries to his legs.

Local police were told the man had experienced some issues with the brakes of his truck and pulled over in his semi-trailer on the northbound lanes of the motorway.

It was when he'd alighted from the vehicle and inspecting it that it rolled over him, NSW Police say.

The man has gone into cardiac arrest and been rushed to Royal North Shore Hospital, NSW Ambulance said.

Traffic is heavy around Pennant Hills Road and all lanes of the motorway have since reopened, a Transport Management Centre spokeswoman said.