A Fire and Emergency New Zealand spokeswoman said firefighters were called to the fire, at Pukeuri, about 3.45am yesterday.
Crews from Oamaru found a domestic dwelling that was well alight on arrival and it appeared to be unoccupied.
The fire was contained to the house, which sustained substantial damage, and the surrounding grass, and had been fully extinguished.
A fire investigator had been called to the scene.
The cause was not obvious and the house being unoccupied made it a little bit unusual, so the fire was being treated as suspicious, the spokeswoman said.
Oamaru Sergeant Tony Woodbridge earlier said the house had been unoccupied for some time.
He encouraged anyone who may have seen anything in the area at the time to get in touch with police.
