While no members of the public have been reported injured, police said 15 “rough sleepers” had spent the night in the building before it burned down.
Police said they have contacted 13 of those people so far.
Speaking at a press conference Friday, Acting Assistant Commissioner Paul Dunstan said two 13-year-olds had “handed themselves in” late Thursday night and were “assisting police with inquiries.”
Dunstan called on “three to four” other “young people present during the fire” to come forward with their parents “and put their side of the story forward.”
The fire burned throughout Thursday night, causing “significant structural collapse” to the building in central Sydney, Dunstan said.
Pictures showed one of the building’s walls collapsing.
“We continue to keep wetting down the hotspots. We will continue for some time to get final extinguishment there,” said Jeremy Fewtrell, Deputy Commissioner Fire and Rescue NSW on Friday.
Fewtrell said engineers were now assessing the building to determine the integrity of the burned-out structure.