Two people dead in multi-vehicle crash on Sydney Harbour Bridge

Two men have died and several people have been injured after four cars and a bus collided on the Sydney Harbour Bridge.

Emergency services were called to the scene about 1:40pm following reports of a multi-vehicle crash.

NSW Ambulance paramedics treated multiple people for injuries; however, two men died at the scene.

Emergency services respond to the multi-vehicle crash on the Sydney Harbour Bridge.  (ABC News)

Police said the bus driver was uninjured and there were no passengers on board at the time.

Superintendent Clayton McDonald said the investigation into the cause of the crash was ongoing.

"I'm mindful at this time the driver [and] both victims who have deceased have not been identified," he said.

He said other people involved in the crash had been taken to various hospitals.

Superintendent McDonald said initial inquiries indicated that one car veered into oncoming traffic.

He said the crash investigation team from NSW Police had finished their investigation.

"We are now working to remove the vehicles from the bridge as quickly as possible, and we'll have that bridge open as quickly as possible for motorists to return home."

Two people died at the scene. (AAP: Dan Himbrechts)

Transport for NSW coordinator-general Howard Collins urged the public to avoid the area.

"As I speak, both south and northbound are closed of the Sydney Harbour Bridge," he said.

"There are diversions in place. There's significant traffic congestion in the area."

Mr Collins said that, even if all lanes reopen, "there will be significant traffic to clear for the afternoon peak".

He said two lanes of the bridge, moving in both directions, had reopened.

The Sydney Harbour Tunnel remains open, but traffic is very heavy so motorists are advised to use an alternative route if possible.

Buses which normally cross the Harbour Bridge are starting and finishing trips at North Sydney and Macquarie Park.