WHAT WE KNOW SO FAR

THE BUILDINGS

* A century-old, seven-storey former hat factory in Surry Hills had been vacant for four months and was known to regularly house 15 rough sleepers - 13 have confirmed they are safe

* A second building, formerly home to Sydney karaoke bar Ding Dong Dang, was damaged significantly by the blaze

* Plans had been lodged with the local council to convert the two brick-and-timber buildings into a 123-room, two-restaurant hotel at a cost of almost $40 million

* Property manager Reuben, who lives in a neighbouring apartment, told AAP he thought the neglected building was a disaster waiting to happen, saying it "looked like a tinder box".

THE FIRE

* More than 120 firefighters from 30 fire trucks battled the blaze, consuming two buildings after it broke about 4pm on Thursday.

* The old factory's roof, floors and parts of its walls collapsed before 5pm, sending red hot bricks tumbling to the streets

* The second building was damaged significantly

* The inferno was a "once in a decade type of fire", Fire and Rescue NSW Acting Commissioner Jeremy Fewtrell said on Friday

* A combination of factors combined to create the "perfect" conditions for a very intense fire, including the structure's age, height and copious amounts of timber framework, Mr Fewtrell said

* The only reported injury was a minor burn suffered by a firefighter

* Firefighters remained on the scene on Friday, dousing hotspots and monitoring charred remains for potential flare ups.

THE INVESTIGATION

* The police arson squad is leading the probe

* Local locksmith Phu Tang told reporters he spotted a group of high school-aged children running from the building

* Two 13-year-olds handed themselves into police on Thursday night

* Police have urged three or four other young people, who may have been inside the building, to come forward with their parents

THE COMING DAYS

* About 50 residents from two apartment blocks remain locked out on Friday as concerns grow about a bulging wall in the factory

* The integrity of the factory's remaining walls is being assessed by engineers from Public Works NSW and plans for re-entry will be put together in coming days

* Impacted residents are urged to register online with the Red Cross for support

* Among those locked out overnight was a bride due to be married in Hobart on Friday, but firefighters accessed her apartment and retrieved her dress

* A large exclusion zone will remain in place for at least seven days