The main street of a Hunter Valley town was left looking like a war zone after a man driving an allegedly stolen truck crashed into four cars and a building, causing a fireball to erupt.
A man is fighting for his life after being trapped in his car when it was hit by the semi-trailer, which was carrying mushroom compost.
The truck driver has been taken into custody and is assisting police with their inquiries.
NSW Police have launched a critical incident investigation into the situation that unfolded in Singleton on Wednesday morning.
Police responded to reports that a semi-trailer was stolen from a petrol station in Murrurundi, north of Singleton, while the driver was paying for fuel about 7.30am.
The fully-laden semi-trailer was allegedly seen driving erratically south on the New England Highway, passing through Muswellbrook towards Singleton.
Police said they did not initiate a pursuit but deployed road spikes to stop the vehicle.
The truck continued up the main street of Singleton, where it erupted into flames about 9am after crashing into four vehicles and a building, leaving a number of people with serious injuries.
A witness told Nine News he saw a plume of black smoke.
"I drove around the back streets to get around and saw a huge fireball and what looked like a car transporter with cars all over the road," the witness said.
NSW Ambulance Inspector Luke Wiseman said eight people were treated and four taken to hospital, including a 64-year-old male who was flown to Newcastle's John Hunter Hospital in a critical condition with head and chest injuries.
A 62-year-old woman was also taken to John Hunter Hospital with pelvic and hip injuries.
Another man, also 64, was taken to John Hunter Hospital with neck, back and chest pain.
Mr Wiseman described the initial scene as "chaotic", involving "a number of distressed people."
Police said a critical incident team from Port Stephens Local Area Command would investigate the circumstances of the crash, including the use of road spikes. That investigation will be the subject of an independent review.
The New England Highway at Singleton is expected to remain closed for several hours and traffic diversions are in place.
Supplied Photo shows the extent of the damage on The New England Highway at Singleton shortly after impact. #Singleton #news pic.twitter.com/0lZtpz2WgE??? Dan Irwin (@DIrwinphotos) October 11, 2017
#UPDATE: Police have launched a critical incident investigation into Singleton truck crash. #LATEST: https://t.co/fPapCihxQF pic.twitter.com/QCUsaUXM2Y??? NBN News (@nbnnews) October 11, 2017
A fireball erupted when a truck slammed into cars & a building in Singleton. Four people being treated for numerous injuries.@nswincidents pic.twitter.com/AskBd6LX9T??? Jessica Rouse (@JessRouse95) October 10, 2017
Emergency crews remain on the scene.