POLICE have revealed that the driver of a truck which flipped on its side and collided with a stationary truck at Junction Hill may have been having a "medical issue".

The crash occurred near the service station at Junction Hill, when the truck coming from Grafton appeared to lose control as it negotiated a bend coming into 50kmh zone on the southern end of the village.

The driver of an oncoming car, Bob Shea, of South Grafton, said he saw the driver "wrestling with the wheel" in the truck, which was swaying and swerving across the road.

Mr Shea said he swerved to avoid the truck and felt lucky he and his partner, Robyn, survived.

The couple were returning to South Grafton after a trip to Brisbane on April 23.

The police investigation is continuing.