Crash on Sydney's M4 leaves man critical, causes significant traffic delays

A man is critical after a multi-vehicle crash on the M4 motorway in Sydney's west.

A truck allegedly collided with another vehicle on the motorway, past the Cumberland Highway, just after 2.50pm today.

One man, aged 40, has been taken to Westmead Hospital with head injuries.

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He is in a critical condition.

9News understands the truck driver was trapped in the vehicle after the collision and was freed by specialist firefighters.

The crash has caused heavy traffic on the M4 at Wentworthville.

One of the vehicles involved has blocked multiple lanes, while another crashed into a barricade on the side of the road.

All westbound lanes were closed, with traffic backed up for 12 kilometres.

They have since reopened, however drivers are still faced with significant peak-hour delays.

All eastbound lanes have also reopened.

"Continue to allow plenty of extra travel time on the approach as traffic is very heavy," Sydney Live Traffic said.

Motorists have been urged to avoid the area.