All lanes have reopened on the Sydney Harbour Bridge after a head-on collision this morning.

The four-car pile-up caused delays at 8.20am this morning. Early reports suggested six cars were involved in the crash.

NSW Police said two cars collided head-on before crashing into two other cars. 

Four lanes were closed on the bridge going in both directions after a driver was trapped inside their car following the crash and had to be freed by emergency crews.

All lanes have reopened but significant delays are still expected as southbound traffic remains heavy and reaches the Lane Cove Tunnel.

Northbound traffic is also heavy and reaches the bridge from the City West Link and Victoria Road.

Two drivers have been treated by paramedics for minor injuries and have been taken to hospital in a stable condition.

Traffic is backed up to Drummoyne in the west and to Artarmon in the north.

Drivers are advised to leave plenty of extra travel time as traffic backs up on both sides of the bridge.

Police and emergency crews are on site.

This is a breaking story. More to come ...