Specialist fire investigator Mike Harrison, of Dunedin, said the damage was so extensive a cause might never be known.
Middlemarch Volunteer Fire Brigade deputy chief fire officer Blair Dale said his crew were still at the scene about 2.30pm yesterday dampening down hotspots while fire investigators worked to determine the cause.
"She was pretty well-involved by the time we showed up," Mr Dale said. "She was out of control."
The toughest part was getting access to water, which they had to pump from a nearby creek, he said. "The community is rallying around."