Emergency services were called to the unit block on St Georges Crescent,Drummoyne,about 6.10pm.
Fire and Rescue NSW said at 7pm the fire had been contained and search and rescue operations were ongoing.
In a statement issued later on Friday,NSW Police said firefighters “located a body inside the unit”.
Officers established a crime scene and an investigation has commenced.
Anyone with information is urged to come forward. A report will be prepared for the coroner.