Update, 6.40pm: A male person has been found dead after a flat explosion in Mosman Park this afternoon.
Emergency services were called to the fire scene at about 4.20pm.
Fire and emergency services personnel said the said the fire was particularly intense.
Police Inspector Paul Kosovich said at this stage police could not confirm whether the explosion was caused by a clandestine drug laboratory but the organised crime squad and the clandestine laboratory squad would be investigating the cause of the fire.
Residents in the building's remaining six apartments were evacuated.
Police do not believe there are any dangerous fumes but have asked residents to remain in their homes as a precaution.
Gibbon Street between Victoria Street and Wellington Street is blocked and Salter Lane is blocked between Gibbon Street and Buckland Avenue.
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