A burning chair caused by a cigarette is believed to have sparked the evacuation of the Whangarei police station.
All police staff and one prisoner being held in the cells were forced from the central city station at about 9.50pm on Thursday.
Northland police Chief Superintendent Russell Le Prou said a woman, who was not under arrest, was being interviewed in a room at the station when the fire started on the top floor. The chair was damaged beyond repair but the wall it was next to did not ignite.
The station's fire alarm activated as did the sprinkler system in the room.
"The system we run means the only sprinklers that activate are isolated to where the seat of the fire is. It worked well," Mr Le Prou said.