Detective Sergeant Trevor Beatson said police were also treating the fire as suspicious, and he had no doubt that someone in the tiny community would know who was responsible.
"This is not the sort of place someone from outside the community would stumble on," he said.
He had spoken to the elderly owners, who were out of the district over the weekend, saying they had been renovating the house over several years, and had not had insurance.
"The neighbours certainly did a good job," he added.
"By all accounts it looked pretty hopeless when they began fighting the fire, but they stuck at it and prevented a lot more damage being done."
Anyone who had any information was urged to contact Detective Sergeant Beatson at the Kaitaia police station, phone (09) 408-6500, or Crimestoppers, anonymously, on 0800 555-111.