Firefighters were called to reports of a house fire at a two story residential property in Joseph St, Gore about 4.20am yesterday.
They extinguished a blaze which left the house extensively damaged.
A Fire and Emergency New Zealand investigator was at the scene yesterday and returned today.
A scene guard was in place overnight.
A police spokesman urged members of the public who saw anything suspicious on Joseph St around the time of the fire to get in touch if they could assist with the investigation.
■ Contact police on 105 and quote file number 230308/4879.