Senior Firefighter Anthony O'Neill, of the Port Chalmers Fire Brigade, said it appeared the fire started in a bedroom at the rear of the house and spread quickly, extensively damaging the whole building.
The Fire Service was called about 7.40am. The rear of the house was engulfed in flames when a crew arrived from Port Chalmers.
Mr O'Neill said four adults and a child were in the house when the fire was noticed, but all escaped without injury.
Fire Safety Officer Barry Gibson said he was treating the fire as "possibly accidental", but had yet to complete his investigation.
All the residents were up and about when the fire broke out, which was probably lucky, he said, as the house was not fitted with smoke alarms.
"It might have been another story altogether if this fire broke in the middle of the night."
