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Fire and Emergency New Zealand was called to Clyde St at 6.17am to find a residential property well ablaze, a spokeswoman said.
Two crews responded and flames could be seen coming from several windows and doors, causing severe damage.
It took firefighters roughly 45 minutes to extinguish the blaze.
Fire investigator Mark Bredenbeck later arrived to ascertain the cause.
It was "most likely" caused by a child’s bunk-bed covering a plug for a heater, he said.
"It’s possibly caused either the plug to become dislodged and spark, or it’s created some heat that has built up and started the fire that way."
The Fenz fire investigator warned against putting furniture too close to any plug, in order to avoid similar events occurring.
Fenz had a busy Saturday morning, dealing with a vegetation fire in the Dunedin suburb of Helensburgh and a roof fire in Queenstown.
Crews were back in action yesterday, when two helicopters were needed to help extinguish a rural grass fire near Roxburgh in Central Otago.
Fenz was alerted to the blaze off Wright Rd, Teviot, by passers-by at 11.35am.
Two ground crews initially responded from Millers Flat and Roxburgh, but the steep terrain made it difficult to access, a Fenz spokeswoman said.
Helicopters were requested and the ground crew used foam to contain the fire.
It was put out by 2.30pm.