A 10-storey building in Sydney's CBD caught alight with fire crews racing to the scene to battle the blaze.
A sheet of scaffolding in the building caught alight and collapsed onto Pitt Street and Alfred Street near Circular Quay about 8.40am on Tuesday, a Fire and Rescue NSW spokeswoman told AAP.
Everyone in the building was evacuated and seven crews were on scene fighting the fire. The cause of the fire is yet to be determined.
The thick plume of smoke that engulfed Circular Quay was visible from across Sydney Harbour on Tuesday morning as firefighters evacuated the area and controlled the blaze. No reports of injuries.
A 200-metre exclusion zone was set up as thousands of office workers watched on.
"It's flaming. It's really high," a NSW Police officer told AAP at the scene.
The building is covered in black mesh netting which was "well alight" when crews arrived, a Fire & Rescue NSW spokeswoman said.
Emergency crews react to the Circular Quay fire. Source: Marton Dobras
Less than two hours later, 16 fire crews managed to contain the blaze.
Thirteen people were treated for minor smoke inhalation, a NSW Ambulance spokeswoman said.
Earlier, gas cylinders from forklifts inside the building caused small explosions, startling people nearby.
Neil Van der Hoek, who works on the 22nd floor of a building opposite the fire, was one of those who heard and felt the explosions.
"We could see flames through the kitchen window, there were two explosions, from gas bottles I think. We could feel the heat and explosion through the window," he told AAP on the street outside.
"The plastic sheeting on the outside caught on fire and dropped down onto a forklift."
Sarah Halliday was on her way to work when she saw the flames.
"There were two workmen trying to put the flames out from the roof with a small hose," she told AAP.
Circular Quay fire. Source: Marton Dobras
"It got bigger as I left and I heard a few explosions as I got farther away."
Everyone in the building was evacuated and NSW Ambulance is advising people to avoid the area because of the smoke.
Trains have resumed normal services at Circular Quay, St James and Museum, after being shut down briefly because of the smoke.
