Special fire investigator Mike Harrison, of Dunedin, said interviews with the occupants - two adults and three children - and the scene examination pointed to the same conclusion.
''The child in the house alerted the other occupants to the fire and they were able to evacuate from the building,'' he said.
''It was a young child.''
The Milton Volunteer Fire Brigade arrived at the blazing house on Spenser St about 7am, he said. The fire was accidental.
''Quite often children will mimic the actions of their parents, so if they see you lighting fires, whether it's in the fireplace or smoking or whatever, they'll try and replicate that,'' he said.
Lighters and matches were ''tools and not toys'', he said, and keeping them out of the reach of children was important.
The fire highlighted the importance of testing fire alarms and replacing batteries if needed, Mr Harrison said. The fast-moving fire had the potential to be dangerous.
''The smoke alarm started beeping ... a day or two previous and the occupants had removed the batteries from the smoke alarm,'' Mr Harrison said.
''They were in the process of putting another battery in, but they didn't quite get there before the fire occurred.''
