Fifty-two people escaped from the building, with at least five rescued from the roof by fire crews.
Five people had been taken to hospital, with two in serious condition.
Fire and Emergency district manager commander Nick Pyatt said the blaze was "the worst nightmare" for crews.
"This is a once-in-a-decade fire for Wellington."
At its peak, 29 fire crews amounting to more than 80 firefighters, attended the blaze, Pyatt said.
Wellington lawmaker Grant Robertson said there were some "extremely traumatized people" at an evacuation centre, who had survived the fire.
"Many people there have lost absolutely everything they had," Robertson said.
