A Traffic and Highway Patrol officer pulled over the Isuzu truck on Vardys Road at Kings Park, near Blacktown, at 10.20 am on Thursday.

Police say the man had a small but unsecured load in the back of his truck as he drove through the industrial area.

The driver, a man aged in his 40s, got out of his truck and attempted to secure his load when police say he possibly stumbled onto the road in a "terrible and tragic accident".

A semi-trailer hit the man, killing him at the scene.

"It appears at some stage the driver has either stumbled, fallen or stepped back into the path of another vehicle," NSW Police Assistant Commissioner Denis Clifford said.

"Unfortunately, despite efforts by paramedics to revive him at the scene he passed away."

There was nothing to indicate the fatal crash site was an inappropriate place to stop the vehicle, Mr Clifford said.

"I understand the vehicle stopped there for up to 10 minutes before this tragedy occurred so there were a number of vehicles that passed."

It is understood an in-car camera inside the highway patrol vehicle, which had only one officer in it at the time, captured the entire fatal incident.

The crash had taken a toll on everybody involved, including the officer at the scene, Mr Clifford said.

"The officer involved is devastated, as I understand is the other driver and, of course, the family of the deceased," he said.

"I can't begin to imagine how they must be feeling at the moment. Our hearts must go out to them."

The officer is being supported by colleagues.

"The officer involved had the full support of the Police Association of NSW," the association's president, Scott Weber, said.

The driver of the semi-trailer truck was taken to hospital for mandatory blood and urine testing but Mr Clifford said it was too early to be talking about any charges being laid.

"At this stage the investigation team will try to piece together every detail they can about what occurred and, in due course, put it to the coroner."

A critical incident investigation, led by the North West Metropolitan Region, will look at the circumstances leading up to the crash, including the police actions in pulling the man over.

Employees from surrounding businesses watched on as paramedics tried desperately to save the man and the visibly shaken driver of the Mack truck was comforted nearby.

"He was tying his load down and another truck rounded the corner and took him out," a man from a nearby automotive shop said.

"We can't understand how that truck driver got out there."

Police have urged witnesses to contact Crime Stoppers on 1800 333 000.

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