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Fire and Emergency New Zealand spokesman Rewai Grace said three crews were called at 2.13pm to the home in Centre St near Glenalmond Cres.
When they arrived they found the home ''well involved'' in fire and another engine was called.
St John spokesman Gerard Campbell said ambulance officers treated a 16-year-old boy at the scene with ''moderate injuries'' before transporting him to Southland Hospital.
It was unknown whether his injuries were caused by smoke inhalation.
Southland Fire Service assistant area commander Dean Chalmers said the fire was caused by an "accident" involving a domestic fire place.
"The young occupant who was in the house at the time who noticed the fire has managed to successfully evacuate the property and has notified the fire service.
"At that point we’ve arrived to find the house quite well involved in fire.”
“The first machine arrived from Kingswell, we’ve had four appliances arrive here through the duration of the fire.
It took the first crew "a good ten minutes to get on top of the fire".
The back of the house was "extensively damaged", but firefighters managed to largely contain it from spreading to the rest of the house.
Mr Chalmers said the fire served as a warning as although there were smoke alarms in the house they did not have batteries.
"If this event had been at night time, it does post serious risks for the occupants of the house, and just reiterates our safety message that every working smoke alarm has to have batteries in it.”