Fire investigator Craig Bain determined the fire was arson.
"It's been deliberately lit in several rooms."
The blaze was not the only thing the firefighters had to contend with in the early hours of Sunday.
Whangarei police Sergeant Neil Pennington said officers were forced to call for back-up when they were surrounded and harassed by Black Power members from a nearby party. It is understood an attempt was also made to break into a police car attending the scene.
Mr Bain is appalled at the fact people would try to hinder those carrying out a community service.
"Firefighting is a dangerous enough job as it is," he said.
He said the fire provided a timely reminder on the safety precautions Northlanders should be taking this winter. "It's that time of year now to make sure your smoke alarms are working.
"With fireplaces, ensure chimneys are cleaned."
Anyone who witnessed the fire or has any information regarding it can contact the Whangarei police on (09) 430 4500 or Crimestoppers anonymously on 0800 555 111.
Meanwhile, police are investigating another suspicious fire in Otangarei, where a blaze was set in the carport of an occupied home late at night.
Mr Pennington said two men were seen running away from 65 Jack St after a fire was set there about 11.50pm on Sunday.
A fire was started on a plastic drain cover, using clothes, at the carport attached to the home, Mr Pennington said.
He said it was fortunate the people in the home discovered the fire and put it out, as it could have set the car alight that was in the carport.
A police dog failed to track the scent of two men seen running away.
