Crews were expected to continue working Friday morning to clear all debris from the Surry Hills fire before investigations into how the fire started can begin.
There are also fears of possible wall collapse.
There are detours along Elizabeth, Randle and Chalmers streets in the CBD.
NSW Fire and Rescue Commissioner Jeremy Fewtrell told reporters on Thursday night that it could take “several hours” to put out the fire completely.
“It will be a very long, substantial firefighting duration, to completely extinguish the fire,” he said.
Hundreds of people were evacuated from the area when fire tore through several levels on Randle Street, near Central Station, shortly after 4pm on Thursday.
Authorities said it was “incredibly lucky” no firefighters were killed, although one did sustain a small burn on his arm.
More than 100 firefighters were called to the scene from multiple stations in Sydney, taking hours to contain the blaze with a combination of air and ground attacks.
Fewtrell said there were “collapse zones” to protect firefighters while they continued to work.
“We have some engineers attending the scene and they will help guide our decisions on safety considerations, particularly the risk of collapse,” he said.
Once the fire has finally been extinguished, FRNSW and NSW Police investigators will enter the scene to determine the cause of the fire.
Both organizations said it was too soon to speculate.
“Our arson investigators along with police forensics will start making those assessments, but that will be at some later point,” Fewtrell said.
Images reveal destruction in Sydney fire
John Duncan of the NSW Police said the matter was still under investigation.
“We will work with your investigators to find out what really happened. It’s too soon.” [to say what sparked the blaze],” he said.
Fewtrell asked anyone with information or video to come forward.