Crews were called to Randle St in Surry Hills on May 25 as a fire tore through a former hat factory and another empty building.

A tight exclusion zone remains over concerns more of the structure could collapse.

NSW Police acting Assistant Commissioner Paul Dunstan said the two teenagers handed themselves in at separate police stations late on May 25.

"They are now assisting police with our inquiries," he told reporters on May 26.

"We are aware of a further three or four other young people that were present during the fire.

"We ask them to come forward with their parents and put their side of the story forward."

The NSW Police Force arson squad is leading the investigation into the fire.

Assistant Commissioner Dunstan said police were aware of up to 15 people who were sleeping rough in the building the night before the fire.

He said 13 had been confirmed as safe and well.

"There are two other people that we'd like to just track down and confirm they are safe and well.

"Detectives are working furiously to try and locate those people."

IN OTHER NEWS:

Nearby residents had been ordered to evacuate as plumes of toxic smoke were sent into the sky.

The blaze drew 120 firefighters and the fire was declared mostly contained by 7pm.

Fire and Rescue NSW Superintendent Adam Dewberry told ABC News Breakfast on May 26 about 50 residents remained out of their homes due to ongoing safety concerns.

"We've still got a problem with two walls that are in precarious positions," he said.

"[The walls] could come down without notice bringing tonnes and tonnes of bricks down and causing projectiles and causing injury, damage or even death."

The extreme heat damaged adjoining buildings and windows continued to fall, adding to the ongoing danger.

Superintendent Dewberry said a plan was being made for demolition of the unsafe structure "safely and in a timely manner".

A specialist fire investigator was at the site again on May 26 but Superintendent Dewberry said it was too early to go into any detail on the cause of the fire.

The only reported injury from the fire was a minor burn suffered by a firefighter.

Police also attended to cordon off the scene and the eastern suburbs light rail was shut down for the afternoon due to the fire.

With Australian Associated Press