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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 fire in the home ''well involved'' and another engine was called.
St John spokesman Gerard Campbell said ambulance officers treated a 16-year-old boy at the scene for moderate injuries before transporting him to Southland Hospital.
It was unknown whether his injuries were caused by smoke inhalation.
Southland Fenz Assistant Area Commander Dean Chalmers said the fire was caused by an ''accident'' involving a domestic fireplace.
''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.''
The back of the house was ''extensively damaged'', but firefighters managed to prevent the blaze spreading to the rest of the house.
-By Sharon Reece, additionally reported by George Block