The fire in the Pall Mall Arcade building in Invercargill on Wednesday started accidentally, an investigator says.
Eight fire crews and a ground unit were called to the building in Dee St just before 1am on Wednesday.
Six people were evacuated and all were treated for smoke inhalation. A 10-year-old boy was taken to hospital for treatment after suffering severe burns to his face and limbs.
Senior Fire Investigator Murray Milne-Maresca said the investigation started yesterday after the building was deemed safe.
Fire and Emergency New Zealand said the fire started accidentally on a bed.
"It was caused by an electrical device which set the bed on fire.
"Investigations are continuing to establish what the device was, but there is nothing suspicious about the origins of the fire."
