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A large building at the back of the mansion is on fire.
Christchurch City Station senior officer Shane O'Brien said a structure was well involved.
There are three fire crews and a command unit at the site. Colombo and Manchester Sts are closed between Bealey Ave and Salisbury St.
Initially it was thought the mansion itself was on fire.
Starnews.co.nz reporter Georgia O'Connor Harding, who is at the scene said: "There is smoke everywhere. Police are cordoning all sides of the building. There are about four fire crews and a command vehicle."
A police spokesman confirmed emergency services are attending a fire on Manchester St.
Employee Jacinta Walker said she could see a lot of smoke coming from the back of the building.
"One of my colleagues went outside and said his eyes watered quite a badly," she said.
"We can just see smoke from the back of the building from where we are."
A staff member at Southern Hospitality Ltd on Manchester St said they can see smoke coming from the mansion.
"We can't see any flames from where we are but we can see a lot of smoke and the fire engines coming in. It definitely looks really bad."
The 118-year-old, 53-room mansion, in the central city, is undergoing a $9.1 million restoration in order to transform the building into a major tourist attraction.