Dramatic Sydney Blaze Consumes Seven-storey Building

More than 100 firefighters battled towering flames and thick smoke Thursday as a huge blaze consumed an unoccupied seven-storey building in central Sydney.

The flames appeared to be dying down as firefighters poured in water from multiple hoses two hours after the fire took hold in a building near the Central Station railway hub.

Shortly after it erupted, flames almost as high as the building itself shot into the sky.

The entire top floor wall leaned over and crashed in pieces into the street below, fire service video showed, as the building glowed orange with flames.

The carcass of a burning van was parked in front.

"More than 100 firefighters, from 20 fire trucks and multiple stations, continue to work to extinguish the blaze," Fire and Rescue NSW said in an update after night fell in the city.

Two buildings had been affected by the fire, it said, both believed to have been vacant.

"Crews have also been dousing smaller fires which threatened to spread to surrounding buildings, including residential apartments."

One firefighter suffered a minor burn on his right arm but did not need hospital treatment, the fire service said.

"Firefighting operations are expected to continue throughout the night," it said.

There were no other reports of injuries from the blaze, which broke out in the afternoon in a building described by local media as a heritage-listed, unoccupied former hat factory.

"The public is urged to avoid the area as firefighting operations continue," the fire service said, as the sound of emergency vehicle sirens echoed through the city streets.

One commuter, Corrin Ellis, told the national broadcaster ABC he was going to catch a tram when he noticed the fire.

"All you could feel when standing here was heat on your face from the flames," Ellis told the ABC.

"And when they started to move us back, a car just in front of the building caught fire," he added.

"The front of the building collapsed and flames just spewed from the top of the building."

New South Wales Premier Chris Minns urged people to "stay safe".

"The public are advised to avoid the area as firefighting operations continue," he said on social media.