The sole tenant, a Ranfurly man in his 40s, escaped the blaze and was later treated by St John staff for smoke inhalation.
The Bute St property was one of two self-contained brick-clad flats, separated by a garage.
Ranfurly Chief Fire Officer James Hazlett said the tenant of the rear flat had ''lost everything'' in the fire.
''I gather he was woken up by a big bang and then got himself out of the flat.''
The man was believed to be staying with friends in Ranfurly yesterday.
He could not be contacted for comment.
Firefighters from Ranfurly and Naseby spent several hours at the property, fighting the blaze.
An elderly woman in the front unit was checked by St John staff but was uninjured and the front flat was undamaged, Mr Hazlett said.
Police officers were involved in the fire investigation but after the scene search was completed, New Zealand Fire Service specialist fire investigator, Mike Harrison, of Dunedin, said ''from our point of view, it doesn't appear suspicious''.
Although he was unable to confirm the cause of the fire, he was ''pretty certain it started in the kitchen''.
The scene search would be finished yesterday but no cause would be announced until the completion of any investigation by insurance investigators, Mr Harrison said.
''We wouldn't want to prejudice the outcome of any further investigation.''
Asked whether there was a smoke alarm in the flat, Mr Harrison said: ''We haven't found evidence of a smoke alarm, but that's not to say there wasn't one.''
Emergency services were alerted by several neighbours about 10.30pm on Monday.
Mr Hazlett said it was the fourth ''serious'' house fire in Ranfurly and Naseby this year.
