Revealed: What caused the huge inferno in Sydney's CBD

Investigators have now determined the cause of a construction site inferno that sent toxic plumes of smoke sweeping across Sydney's CBD.

Seven News can reveal that it was a construction worker’s blow torch that caused Gold Fields House at Circular Quay to erupt in flames, bringing much of the city’s transport to a standstill.

In the heart of peak hour, in the centre of the city, a high-rise building already under demolition was taken down a little faster when the massive blaze broke out.

Thousands of office workers poured into the streets as a number of buildings around the site were evacuated, while ground-level smoke also caused the closure and evacuation of Circular Quay railway station.

The fire is thought to have erupted on a lower level caused by stray embers from a blow torch cutting through steel.

It’s understood the sparks ignited a protective plastic mesh around the scaffolding, which melted and spread the flames.

Neil Van der Hoek, who works on the 22nd floor of an opposite building, heard the startling explosions.

"We could see flames through the kitchen window, there were two explosions, from gas bottles I think. We could feel the explosion through the window," he said.

Fifteen fire crews, including specialist HAZMAT and ladder-units, battled the blaze for nearly two hours.

Twenty-four construction workers at the Pitt Street building were evacuated with 13 treated for minor smoke inhalation on the scene by NSW Ambulance.

A 200-metre exclusion zone was established as a low-level toxic smoke spread in the air from the burnt plastic netting, and while authorities assessed the threat of building collapse and explosions from gas cylinders inside.

"The fire had some challenges in that there were a number of gas cylinders on the building site that not only exploded during the height of the blaze but (others) were also affected by the heat," Fire & Rescue NSW Superintendent Andrew Ticehurst said.

"There was some significant dangers to the firefighters with the gas bottles exploding, as well as it being a building being under demolition, there (were) some concerns around how stable the building (was)."