Two men killed in four-car and bus crash on Sydney Harbour Bridge

Dashcam footage has shown the moment a car veered into the wrong lane on the Sydney Harbour Bridge, causing a pile-up that killed two people.

Two people were killed when four cars and a bus crashed at the southern end of the Sydney Harbour Bridge at about 1.40pm.

NSW Police said initial inquiries suggested a car headed northbound veered into the southbound lane, causing an initial crash, that also led to the other two cars and bus colliding.

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Dashcam footage shows a small hatchback veering into oncoming traffic before it collides with a car heading in the other direction.

Two men died at the scene.

Major operation for emergency services

NSW Ambulance Superintendent Jordan Rosser confirmed that two other patients, a man and woman, were transported to Royal North Shore Hospital in stable conditions with minor injuries.

Police, firefighters, nine ambulances and a CareFlight chopper attended the crash.

Firefighters assisted two people stuck in their car after the crash.

"Two patients were trapped in the vehicle ... they cut and pulled apart the car, removed both doors," Fire and Rescue NSW Superintendent Adam Dewberry said.

"[Firefighters] started providing fire protection, and cleaning up the significant wreckage all over the road due to the collision."

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Sydney City Police Acting Commander, Acting Superintendent Clayton McDonald said the investigation into the crash was ongoing.

"Initial inquiries suggest that the cause of the collision ... came from one vehicle travelling in the northbound lane," McDonald said.

"[It] veered across into the traffic in the southbound lane resulting in the initial collision.

"And a number of other collisions at the time collisions"

The bus was driving in the bus lane when the accident occurred.

The bus driver was not injured and no one else was on the bus, as it was not in service.

Warnings for motorists to avoid the area

All lanes of the Harbour Bridge are now open, however, motorists are still being urged to avoid the area.

"There are significant traffic congestion in the area ... there will be significant traffic to clear in the afternoon peak," Transport for NSW Coordinator-General Howard Collins said.

The Sydney Harbour Tunnel remains open in both directions but traffic is heavy.

The train and metro services are still running.

A motorcyclist also allegedly rode through the crime scene while emergency services were dealing with the situation.

He was arrested at the scene and taken to Day Street Police Station.