"As a result of that the occupants weren't alerted."
He said the residents' were oblivious the far side of their unit was ablaze until they heard the commotion on the street outside and voices of firefighters and police telling them to get out.
It was also complicated by a complex top-floor layout that took emergency services time to navigate as they went door-to-door evacuating sleeping residents.
The historic building did not have a central alarm system and relied on individual smoke alarms in flats and corridors.
Fire safety officers would today speak with the building's owner about the need for working smoke alarms in rental accommodation.
A law change last year made smoke alarms compulsory in all rental homes. Landlords and tenants are responsible to keep alarms working, and tenants have to replace expired batteries while they live at the property.
Tenants who damage, remove or disconnect a smoke alarm - including taking out batteries - is now breaking the law and may be fined up to $3000.
Four fire engines and one turntable ladder were needed to fight the fire at the back of the building.
The blaze in the outer wall cavity was not being treated as suspicious.