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Fire crews were called in to battle the blaze at the back of the mansion between Manchester and Colombo Sts on Tuesday afternoon.
Fire and Emergency New Zealand specialist fire investigator Mike Gaskin was at the scene on Wednesday morning to determine the cause of the blaze.
He said it was "certainly suspicious."
"It's an unoccupied building and they don't often set fire to themselves."
Christchurch City Station senior officer Shane O'Brien said the structure was well involved when fire crews arrived about 3.30pm.
Three crews and a command unit attended. Colombo and Manchester Sts were closed between Bealey Ave and Salisbury St for a short time.
Initially, it was thought the mansion itself was on fire.
The 118-year-old, 53-room mansion was undergoing a $9.1 million restoration in order to transform the building into a major tourist attraction.