Jackknifed ... the petrol tanker on the bridge. Photo: Dallas Kilponen

A CareFlight helicopter landed at nearby Observatory Hill with a doctor and a paramedic, who treated four people - a woman in a critical condition and two people in a serious condition.

The accident, which took place about 1.20pm today, involved an empty petrol tanker heading south and two cars, say police.

CareFlight spokesman Ian Bedham said the 48-year-old woman, 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.

The crash scene on the bridge ... by Twit Pic@ShayNarsey.

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.

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

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

Point of impact ... The tanker and the car, pictured by Ian Abbott.

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

There is also a white ute involved.

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

The southbound lanes are travelling slowly through towards the city. The tanker’s nose is almost at the fence that separates the roads from the railroad line. There are two hatchback cars that are nose to nose to each other on the bridge.

Patrick Allaway was driving north over the bridge at about 1.15pm, when the truck and the car 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 jack knifing 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.’’

Emergency crews are 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 truck 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 have been advised to find alternative routes - including the Anzac Bridge and Gladesville Bridge.

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