Mr Gibson, of Dunedin, said ashes in a plastic bucket ignited on the front deck of an A-frame house in Henry St.
The wind was blowing towards the house and the fire spread quickly, he said.
The house was built back ''off the road'' so the fire was ''well involved'' by the time the Fire Service was called at 12.45pm, he said.
The second floor had ''severe'' fire damage and the ground floor had heat, smoke and water damage.
It was fortunate the fire ignited in the daytime when nobody was inside, Mr Gibson said. If it had ignited when the family was asleep, then it could have been a tragedy.
Ashes should be put in a metal container and left for at least five days before disposal.
Ashes put in a plastic bucket, cardboard box or wrapped in newspaper was a ''common practice'' for many, Mr Gibson said.
''This happens far too often. It's just a disaster waiting to happen.''
Waikouaiti Deputy Chief Fire Officer Stewart Paul said the fire was ''well involved'' when three crews and three appliances from Waikouaiti and Palmerston stations arrived.
The crews had the fire ''under control'' in 20 minutes, he said.
