The fire is thought to have started in a bag of clothes and shoes which were sitting on top of a disused washing machine near the garage area.
He said it appeared someone had entered the house through an open door on the ground level and started the fire.
The fire burnt a wall and had started to scorch the ceiling.
"If they didn't have a smoke alarm the outcome could have been very different," Mr Clayton said.
Fire investigator Terry Baylis reiterated the need for functioning smoke alarms.
"Had there not been smoke alarms that fire could have quite easily broken through the floor and the whole house would have gone. If the place had filled up with smoke it could have been fatal," he said.
It was a reminder to have an escape plan in place.
"If you have to get out at 2am and the room is filled with smoke it's vital you know where you are going."
There was no evidence the fire had started from an electrical fault.
Police appealed to residents in the area to report any suspicious activity they may have noticed or other fires that may have been lit. Contact Mr Clayton at Whangarei station on 09 430 4500 or anonymously on 0800 Crimestoppers 0800 555 111.