The cause of an electrical fire that shut down parts of Sydney's CBD and sent thick black smoke billowing into the streets is still unknown.
Firefighters were called to the semi-demolished Sydney Water building on Bathurst Street after a fire broke out in an electrical substation just before 10am (AEDT) on Wednesday.
Hazmat officials are monitoring air toxicity and masks were distributed to emergency workers.
"The fire has been knocked down," Superintendent Peter Stathis said at the scene.
"We're having an aggressive firefighting attack on the ground floor."
The ground floor had partially collapsed into the basement, he said, adding that it wasn't yet known what sparked the blaze at the base of the 26-storey building.
Busy streets had to be closed but it was hoped they would reopen before peak hour.
The fire had been "tricky" to contain because firefighters had to isolate the power before moving in, Mr Stathis said.
Crews then had to use carbon dioxide gas and water mist, as well as isolate an area containing asbestos.
There had also been fears of smoke or water penetrating nearby Town Hall railway tunnels, but Mr Stathis said state authorities had not reported any breaches.
One hundred firefighters were called in to battle the blaze, with groups rotated in five-minute intervals due to the intense heat.
This heat rendered thermal-imaging devices useless and one firefighter was treated for heat exhaustion.
Bathurst Street was closed between George Street and Castelreagh Street.
Gregor Greid, who works at the Ted's Camera store on Pitt Street, said he'd been waiting behind the cordon for two hours after police told him to shut up shop around noon.
Other workers were ordered to stay in their buildings.
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