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At its peak, seven fire crews battled the blaze at the rural Seal Point Rd property, near the corner with Highcliff Rd at Sandymount, in the early hours of April 13.
The fire eventually consumed the southern wing of the 250sqm home. The wing consisted of bedrooms and a garage containing several vehicles.
Firefighters at the scene initially suspected the blaze was suspicious and police were notified.
However, an incident report released under the Official Information Act by Fire and Emergency New Zealand found no evidence of any accelerant in the smouldering wreck of the home, nor any suggestion it had been broken into.
The ''total destruction'' of the upper floor of the south wing of the house meant investigators were unable to pinpoint the fire's exact origin, the report said.
While they found the cause of the fire could not be determined accurately, they were able to conclude on the balance of probabilities its origin was accidental.
The investigators believed the fire could have been caused by an electrical storm starting a slow smouldering fire, which eventually erupted a full day after a lightning strike.
''It is known that lightning was prevalent the night before the fire and it is possible a strike had hit the roof of the south wing,'' they wrote.
''If this was the case the lightning, in taking the path of least resistance to ground, may have started a smouldering fire in the house which remained unnoticed for 24 hours.
''Unfortunately, this cannot be proven at this stage.''
Property owner Trevor Heaney said yesterday his insurance company, FMG, had ''come to the party'' and he planned to rebuild.
''There's been a lot of pulling down burned wood.
''We're planning to rebuild, but it won't be quite so big.''
The same property was the site of another blaze in September 1993, when a rental home on the 32ha property was destroyed by fire.
Former Otago Peninsula Senior Constable retired Lox Kellas said earlier this year the cause of that fire was not believed to have been suspicious.