Lanes reopen after truck fire causes traffic chaos on major Sydney road

All lanes have reopened on Sydney's congested Anzac Bridge after a truck caught on fire.

A passerby motorist alerted a truck driver that the back of his vehicle was ablaze out of the M4 tunnel about 10.25am today, Fire and Rescue NSW said.

The truck pulled over, shutting down three citybound lanes.

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All lanes have since reopened as firefighters have extinguished the flames.

A tow truck was on-site to move the burnt truck while emergency services cleared the debris from the road.

Heavy traffic built up on citybound entries to the Anzac Bridge, including the Rozelle Interchange, causing lengthy delays, Transport Management Centre said earlier.

Traffic has since returned to normal.