All lanes have been reopened on the Sydney Harbour Bridge after a multi-vehicle pile-up this afternoon.

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The accident, which took place about 1.15pm today, involved an empty petrol tanker heading south and two cars, police said.

A CareFlight helicopter took off at 1.23pm four minutes after being notified about the crash from NSW Ambulance, and landed at nearby Observatory Hill with a doctor and a paramedic at 1.34pm, spokesman Ian Bedham said.

They treated four people - including a 48-year-old woman in a critical condition and a man and a woman, both aged 20, in a serious condition.

CareFlight spokesman Ian Bedham said the woman, 48, from North Ryde, who had been going north had a severe head injury and was taken to Royal North Shore Hospital in a critical condition.

He said three other people, who were able to get out of their cars by themselves after the accident, were treated on the deck of the bridge by paramedics and taken to hospital.

One victim, the 20-year-old woman, was taken to Royal Prince Alfred Hospital with serious chest injuries, NSW Ambulance said.

The man had suspected spinal injuries and was also taken to  Royal Prince Alfred Hospital in a serious condition.

On the bridge

The tanker, owned by Ron Finemore Transport, appeared to have jackknifed across the northbound lanes. One of the cars is a "No Birds" rental car.

The tanker ruptured a water main which ran alongside the train tracks and water was gushing out onto the road.

One of the hatchbacks was very badly damaged and its front was completely crushed in.

There was also a white ute involved.

There were about 40 emergency service personnel - from firefighters to paramedics - on the bridge.

All of the northbound traffic was being turned back via the southbound lanes. Traffic was backed up least through the Kent Street, back towards the Anzac Bridge.

The tanker's nose was almost at the fence that separated the roads from the railroad line. There were two hatchback cars that were facing each other on the bridge.

'How did it miss me?'

Patrick Allaway was driving north over the bridge at about 1.15pm, when the truck and the two cars in front of him collided.

Mr Allaway said it was unclear whether it was the car or the tanker that crossed into the wrong lane.

"But it looked like the truck came across and there was considerable damage to the car, the driver's side of the car was badly crushed.

"There was a huge bang, in fact I heard the noise first and [there was] lots of debris in the air."

Mr Allaway said the truck then jackknifed, blocking all north lanes and stopping about 50 metres away.

"The truck stopped probably 50 metres further down the road so [it] was sliding and jackknifing and stopped on the northerly side of the bridge."

Other drivers slammed on their brakes to avoid the collision, resulting in a few other crashes nearby, Mr Allaway said.

"Certainly the car in front of me went into the car in front of it, so it wasn't able to stop in time."

He said it was lucky there were very few cars heading north at the time, because they could have been hit by the jackknifing truck.

"It was a real shock. I'm still sitting in a park on the other side of the bridge going 'how did it miss me?'.

"It's my lucky day."

Petrol tanker was 'empty'

Emergency crews were planning to tow the tanker away. Drivers stuck in their cars waiting to get on the bridge to go northbound were walking towards the accident site to take a closer look at what happened.

Dave Coleman, a spokesman for Ron Finemore Transport, said the tanker was empty and returning from a major petrol delivery when the accident happened.

He said his company had been in touch with their driver but they were not yet able to find out how or why the accident took place. The driver was taken to the Royal North Shore Hospital, Mr Coleman added.

Motorists were advised to find alternative routes - including the Anzac Bridge and Gladesville Bridge.

Buses were not running across the bridge and passengers were advised to take trains to and from North Sydney station instead.

Meanwhile, Transport NSW report that it is too early to say whether an incident in which a man died on the  tracks at Central Station at 12.50pm will impact peak hour services.Traffic is backed up least through the Kent Street, back towards the Anzac Bridge.All of the northbound traffic is being turned back via the southbound lanes.Traffic is backed up least through the Kent Street, back towards the Anzac Bridge.