Fire risk management officer Stuart Ide suspected that had been the case, but yesterday went back to the Farnham St house at 7.30am - about the time it was believed the fire started - to make sure.
The original mirror had been face up on a dresser in front of an east-facing window which would have picked up early morning sunlight.
Yesterday, conditions were similar and he set up another mirror as close to the same position as he could on the dresser.
It concentrated the sunlight to a pinpoint, like a magnifying glass, and in about 10 to 15 minutes some material at the point of focus had started smouldering.
Mr Ide believed Wednesday's fire started in some sort of combustible material on the dresser, such as paper or cloth.
The window above the dresser blew out when an aerosol can exploded, which also spread the fire throughout the room.
''From there, it just got bigger,'' he said.
The occupant of the flat, Sarah Hoogenraad, was not at home when the fire broke out.
The bedroom had major fire damage. The entire house and contents were smoke damaged.
Dunedin man Barry Oats contacted the Otago Daily Times after yesterday's report to say he had had a similar experience about five years ago.
His wife phoned him at work to say she could smell smoke in the house and he told her to switch off the electricity and headed home.
He found a make-up mirror on a bedroom floor which had left a black mark on a cardboard box, even though the day was partly cloudy.
The following day, at the same time, he
set up the mirror exactly as it had been and it concentrated sunlight on the cardboard box.
Before long, the box had started to smoulder, he said.
