Kerikeri and Ōkaihau brigades also responded with volunteers relieved by a paid crew from Whangārei at 6am.
The two-storey A-frame house was home to a woman and two grandchildren, aged around 3 and 8.
The Advocate understands they left earlier that night after a man made a threat to burn the house down.
Detective Inspector Rhys Johnston, of Northland CIB, said police were ''following strong lines of inquiry''.
Officers had been going door to door talking to neighbours and were examining CCTV footage.
He could not comment further while the investigation was continuing.
A neighbour, who did not want to be identified, said three of his windows had cracked from the heat and guttering had melted. His car had also been damaged.
He was woken up by a nephew from nearby Williams Rd banging on his bedroom window.
Another family member, who was ''a bit of a night owl'', had seen the flames and raised the alarm.
''I walked outside and saw the house was completely ablaze. I called 111 but they said the brigade was already on their way. I was worried about the people in the house until I saw her car was gone.''