The pantech truck, which was carrying a load of food, tipped over near the intersection of Warringah and Pittwater roads in Dee Why at 6.10am and slid along the busy road, before coming to rest on top of a car.

A man and woman who were inside the car were trapped in their vehicle for more than an hour before they could be freed from the wreckage. Police said the woman was flown to hospital with serious injuries, while the man's injuries were not life-threatening.

A NSW Police spokeswoman confirmed both the injured man and woman were police officers from the Central Metropolitan Region. It was not clear if they were on duty at the time.

Four other men were taken to hospital with injuries that were not life-threatening, police said.

Joel Hatton said his car was just metres from being wiped out as the truck ploughed through the busy intersection.

He said he jumped out of his car and thought he would find people dead.

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

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.

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 freed. 

Dee Why resident David Gorrick had pulled up in his car at the traffic lights when he saw the truck 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 of 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.

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."

This is what's left of the car which was stuck under the #deewhy truck. Female police office was pulled from it alive pic.twitter.com/8ENaeZs8Qi — Daniel Sutton (@danielsutton10) October 21, 2014

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."

Police said the pantech truck lost control as it turned from Warringah Road into Pittwater Road and crashed into eight other vehicles. The cause of the crash is under investigation.

A NSW Police spokeswoman said five men and one woman, all believed to be aged in their 30s and 40s, were injured in the crash. The men were taken to Royal North Shore Hospital, and the woman was flown to Royal Prince Alfred Hospital.

Police said the truck driver had undergone mandatory testing, and the truck would remain at the crash site until it underwent a mechanical examination.

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 much of the morning.

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.

Dee Why crash from the ATN chopper - Pittwater Rd sthbnd at Warringah Rd @gettrafficnsw pic.twitter.com/OcO8vQJf2U — Paul Latter (@paullatter) October 20, 2014