Hoses poured water down the building, all of its windows lit by flames.
More than 100 firefighters battled towering flames and thick smoke from a seven-storey blaze in central Sydney on Thursday as the fire service warned it was spreading.
"The building is starting to collapse, while the inferno is beginning to spread to several neighbouring buildings, including residential apartments," Fire and Rescue NSW said in a statement.
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.
Hoses poured water down the building, all of its windows lit by flames.
The carcass of a flaming van was parked in front.
Images posted on social media showed flames almost as high as the building itself shooting into the sky.
Emergency services said they deployed more than 100 firefighters, with 20 fire trucks, who were "working to contain and extinguish the blaze" on Randle Street near the city's Central Station.
"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.
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