A car believed to have been drag racing has crashed into the back of a safety truck trying to clear a duck from the road in Perth's Graham Farmer Freeway tunnel.

The safety truck, complete with flashing lights and warning signs, was helping Main Roads remove the duck from the tunnel just before midnight.

The car collided with the traffic hazard warning sign in the tunnel. ( ABC News )

The speed limit on that section of the freeway had been reduced to 40 kilometres per hour, from its usual 80kph.

WA Police said other cars also travelling at high speed through the tunnel at the same time failed to stop.

A man and a woman in the car that crashed are in Royal Perth Hospital's trauma unit in a serious but stable condition.

Police said their injuries were not life-threatening.

They hope surveillance footage from security cameras in the area will help identify the other vehicles involved in the incident.

A Main Roads spokesman confirmed the duck was recovered safe and well from the tunnel.