"There was a slight breeze, sufficient for the embers to blow around, probably catch hold of some fine combustibles like leaves. Then it's spread up the side of the building."
If insurance company investigators reach the same conclusion, the fire will be formally marked down as the 12th fire in Otago caused by the disposal of hot ashes since last July and the second in Queenstown in five weeks.
"It's accidental but it's a concern."
It is understood the property's owners live in Singapore and rent it out through website Airbnb.
On Saturday, as fire crews dampened down and checked for hot spots, a group of five Asian people arrived at the property. They were spoken to by police and then shown to the property.
Ide said the fire service circulates safety leaflets to rental agencies that manage flats but that could be extended.