The two 13-year-olds reportedly handed themselves into separate police stations in Sydney on Thursday night, as 120 firefighters from 30 fire trucks battled the blaze which broke out late yesterday afternoon.

Three ladder trucks were used along with tenders on the ground to hose down the intense fire which sent a huge plume of dark smoke across the city, while spot fires were extinguished at adjacent apartments.

The building’s roof collapsed, then the floors, before parts of the building’s walls fell down, sending red hot bricks tumbling to the streets below.

Firefighters worked through the night to extinguish the blaze and are using a drone to monitor hot spots for potential reignition.

Of greater concern is the “very good chance” two walls will fall “without notice”.

“There are two walls that are in a precarious position,” Fire and Rescue NSW Superintendent Adam Dewberry said.

“We are talking about tonnes and tonnes of bricks that could come down and become projectiles.

“That is why we have such a tight exclusion zone, not allowing anyone in including firefighters until we get the engineers in.”

Once the area is safe, residents and businesses are expected to be allowed to return to their premises and the major thoroughfare Elizabeth Street will be reopened.

Among those locked out overnight was a bride getting married on Friday.

Firefighters were able to enter her apartment and collect her wedding dress during a run for essentials.

The fire closed nearby streets and forced the rerouting of buses and the shutdown of the nearby light rail service, affecting travel for thousands of commuters.

-with AAP