Fire Investigator Colin Melville was unable to confirm exactly how many people but said it was understood that most, if not all, were outside near the barbecue when the fire started.
One person was treated for burns by Hato Hone St John paramedics and taken to Whangārei Hospital in a moderate condition.
Firefighters eventually left the scene at midnight. However, at 3am today the brigade was back at the property as the fire had reignited.
Little had been left of the house, Smith said.
A scene examination carried out by Melville this morning determined the fire had been accidental.
He said the fire started when burning fat that dripped from a BBQ set the deck alight.