Central North Otago fire risk management officer Stuart Ide said the fire, at Aspen Grove, Fernhill, on Sunday afternoon, took hold after hot ashes in an unsealed metal bucket on a deck were whipped up by the wind.
He said it was the third in Central Otago in two weeks caused by the poor disposal of hot ashes from wood burners or fires.
Mr Ide, who led the investigation, said the wind had been swirling and picked the ashes up and set alight some ''other combustibles'' on the deck.
''We don't know what those combustibles are; it could be pine cones or the firewood there. Then it has taken hold.''
Mr Ide said the other two fires were at Clyde and Galloway, both near Alexandra. One was caused when ashes were put in a wheelie bin.
''Quite simply, you've got to put them into a metal bucket or drum, with a lid on it to exclude the oxygen, or cool the ashes straight away with water.''
Twenty-four firefighters from Queenstown and Frankton tackled Sunday's blaze.
It took hold in one house about 1.15pm, before spreading to the other.
Two people escaped from the second property without injury.
No-one from the other house was at home.
Mr Ide said everyone should be more cautious at this time of the year.
People should also be wary of overloading electrical multi- boxes.
And they should not attempt to extinguish a fire themselves, unless trained.
''The advice is always to get out of the property and call us as soon as possible.''
Mr Ide said the investigation was concluded and insurance assessors, who attended yesterday afternoon, agreed on the cause.
