A woman is in a critical condition after a multi-car pile-up on the Sydney Harbour Bridge that has caused traffic chaos across the city.

Three cars, a ute and a petrol tanker crashed shortly after 1pm (AEDT) at the southern end of the bridge.

The empty petrol tanker jack-knifed, forcing authorities to close all lanes of the bridge, but they were reopened about 3pm.

The 48-year-old woman was left unconscious and trapped in the wreckage.

CareFlight director Ian Badham says she was pulled from the car and placed in an induced coma.

She has been flown to Royal North Shore Hospital with severe head injuries.

Two other people have been taken to Royal Prince Alfred Hospital (RPA) in a serious condition.

One of the patients taken to RPA is a 20-year-old woman with serious chest injuries and the other is a 20-year-old man with suspected spinal injuries.

The petrol tanker was sitting horizontally across five lanes of the bridge, but was not posing a flammable threat. Two cars were sitting nose-to-nose nearby.

Heavy traffic

Traffic quickly banked up after the crash. ( Twitter: @mattgreen_aus )

Motorists are being told to take the Gladesville Bridge or Anzac Bridge to cross the harbour as there is very heavy traffic through the Harbour Tunnel.

Bus passengers can use their tickets on trains, with southbound buses terminating at North Sydney Railway Station and northbound buses taking the tunnel.

Trains continue to operate on the bridge and bus passengers can use their tickets to catch a train or ferry.

There are reports traffic has stretched back over the Western Distributor in the south and Miller Street in the north.

Deputy Opposition Leader Linda Burney has driven past the crash site.

"There is an enormous amount of spectators building up, lots of cameras and the oil tanker is very much jack-knifed in the side of the bridge," Ms Burley said.

"There is one car that's very, very smashed up and a number of other cars have bits and pieces knocked out of them, but it's a bit of a mess I can tell you."

Commuters are also likely to be affected by disruptions to train services at Central Station.

Acting assistant commissioner Mark Walton told reporters the disruption was caused by a police operation.

A man died on the tracks after an apparent attempt to harm himself on a railway platform.

The scene of carnage after the tanker and four cars crashed early on Friday afternoon. ( Twitter: @mab397 )

ABC/AAP