Three people were injured in the crash that caused a car to flip and burst into flames, with the ensuing carnage forcing the bridge to close during the busy morning peak hour.
Dashcam vision posted online that captured the accident shows a black SUV heading north colliding head-on with a white van, before the SUV flips and lands on its roof before exploding into flames.
In the seconds after the impact, bystanders can be seen running towards the black SUV and the white van to help the trapped drivers.
NSW Ambulance Inspector Giles Buchanan said a number of bystanders and other emergency service workers risked their lives to help.
"(They) are absolute heroes, we can't thank them enough," he said.
"One of the vehicles was well alight when we arrived on the scene, it was an extraordinary effort to get the patient out of the vehicle safely."
The fiery crash caused traffic chaos with the bridge - the main arterial road into the CBD from the north - forced to close in both directions for more than an hour after the crash, which happened around 7am on Monday.
Live-streamed images showed flames stretching metres into the air from the blackened, up-ended vehicle in lanes five and six at the CBD end of the bridge near Circular Quay.
Six ambulances were called to the scene, along with police as well as NSW Fire and Rescue.
Police Rescue worked to free a man believed to be in his 20s who was trapped in his van.
He suffered serious injuries to his pelvis and was taken to St Vincent's Hospital in a serious but stable condition.
The drivers of two other vehicles, a man believed to be in his 30s and a woman believed to be in her 60s, were treated at the scene by paramedics.
The man in his 30s suffered a suspected head injury and was taken to Royal North Shore Hospital in a serious condition while the woman suffered minor injuries and was taken to Royal Prince Alfred Hospital.
NSW Fire and Rescue Acting Superintendent Matt Sigmund said upon arrival crews saw "large volumes of thick black smoke coming from one of the vehicles".
"Thankfully, the person in that vehicle managed to get themselves out safely and they were treated by NSW Ambulance," he told the Nine Network said.
A second driver was also trapped and NSW Fire & Rescue worked to protect Police Rescue from the flames as they worked to extricate that driver.
An investigation into the crash is underway.
Emergency services remained on the bridge until 8.20am and drivers were urged to avoid the entire city centre or delay their trip.
Two of three northbound lanes have reopened, and two of five southbound lanes are also now flowing, after all lanes were earlier closed.
Some buses have been diverted away from the bridge, although train services have continued across the bridge.
Traffic is heavy throughout Sydney on "all major routes" approaching the bridge, and anyone travelling through the city should allow plenty of extra time.
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