An out-of-control truck has overturned at a busy intersection in Sydney's north, trapping at least two people in their car and spilling fuel across the road.

One witness said the truck lost control on a hill before overturning and crushing a number of cars that had stopped at traffic lights at the intersection of Warringah Road and Pittwater Road in Dee Why about 6.10am on Tuesday.

Fire and Rescue NSW Superintendent Paul Bailey said firefighters rescued six people from their vehicles, including a man and woman in the same car who were trapped beneath the truck. The pair were still trapped in their vehicle but talking to rescuers just before 7am.

The man was soon rescued, but the woman remained trapped until about 7.40am when she was pulled free. Superintendent Bailey said the woman was suffering "serious injuries". She was flown to Royal Prince Alfred Hospital for treatment.

Police said a total of six people were taken to hospital with a range of injuries. Witnesses said it was remarkable no one was killed in the crash.

Dee Why resident David Gorrick had pulled up in his car at the traffic lights when he saw the truck, which police said was carrying food, hurtling down Warringah Road towards him.

"I was stopped at the lights and the truck came down Warringah. It tried to take the corner, it obviously lost its brakes, it was on two wheels and it slid across the road," Mr Gorrick, who was on his way to work, said.

The vehicle in front if his, a ute, was hit by the truck and the driver had to kick his way out, Mr Gorrick said.

"I got out to help people. There was petrol flying around," he said.

Mr Gorrick said the truck driver was "very shaken up" and had a bleeding nose.

"It was very quick. I was lucky," he said.

A taxi driver who saw the crash called radio station 2GB to say that the truck had travelled down the hill on Warringah Road before hitting the median strip, crossing to the wrong side of the road and overturning.

The witness said the truck landed on top of three cars that were waiting at the lights.

"They were standing still at the lights, they had nowhere to move, and [the truck] has just gone straight over the top of them all," the witness said.

At least eight cars are believed to have been struck in the crash.

Another witness, Joel Hatton, was just metres from having his car wiped out by the overturning truck.

"There was a big bang. I was just scared, freaked out, it was full on," Mr Hatton, who lives in Dee Why, said.

He said he jumped out of his car and thought he would find people dead. He estimated the truck was travelling at about 80km/h down the hill.

"The truck driver was OK, he climbed out the window, had blood on his face," he said.

Ashleigh Connell, who lives in a house across the road from the crash site, said she heard screeching and a huge bang. She ran outside still wearing her pyjamas.

"People were yelling and swearing. There were people trying to climb into the truck to pull the driver out. All the people from Fitness First (opposite the crash scene) were running out," Ms Connell said.

"It's so bad here. You hear accidents all the time. Literally, there's one every day. When you're watching the TV every night you hear screeching but this one is by far the worst I have seen."

One man, Johnny, was near the intersection when the crash occurred and said he left his car to try to help.

"There were lots of people. Everyone was panicking and trying to help. People were running out from around the place," said Johnny, who did not want to give his last name.

"I would have got here one minute after it happened. I was scared but I guess it was the adrenaline."

A NSW Police spokeswoman said a truck and a "number of vehicles" were involved in the crash.

"It is understood people are trapped, and fuel is leaking onto the road from at least one vehicle," the police spokeswoman said.

A rescue helicopter landed at the crash scene just before 7am.

The same intersection was the site of an almost identical truck crash in 2000, in which an 11-month-old boy died.

In that crash, a tip truck crashed into nine cars that were waiting at traffic lights on Warringah Road during the morning peak hour. A witness, former Australian rugby league player Don McKinnon, reached into a burning car to rescue the trapped boy, but the boy later died from extensive burns.

Following Tuesday morning's crash, a Transport Management Centre spokeswoman said all south-bound lanes of Pittwater Road had been blocked at the accident scene approaching Warringah Road, and were expected to remain closed throughout the morning peak.

Motorists have been advised to use Wakehurst Parkway or Mona Vale Road as an alternative route.

The spokeswoman said the accident scene was spread right across the road, and motorists were experiencing significant delays in all directions.

"An extensive clean-up operation is required at the accident scene. As a result the road is expected to remain closed for some time," she said.

She said a contra flow was in place at the accident site, meaning motorists travelling in both directions could pass the crash scene using the lanes of the north-bound carriageway. 

City-bound buses are delayed up to 30 minutes, and other services on the northern beaches are also experiencing delays.