A Fire and Emergency New Zealand spokesman said fire crews from Gore and Mataura were called to Clyde St at 3.40am on Saturday.
Upon arrival, they found the vacant house well ablaze.
Crews stayed at the scene until 5.15am, and police also attended.
Fire investigator John Smalls said because the house was vacant and the fire started in the middle of the night, it was initially being treated as suspicious.
However, after working at the scene on Saturday, he believed the cause was probably a heat pump at the back of the house.
It was a good reminder for people to ensure they maintained their heat pumps and cleaned out any dust, he said.
His investigation was continuing.
The owner of the house had been planning to demolish it and rebuild on the section anyway, and because the wreckage was unsafe, the demolition was brought forward to Saturday.
