The 110 year-old Antonio Hall on Riccarton Road was partially destroyed by fire last night, and required a full-scale firefighting operation involving 50 personnel and air support.
No one was injured in the blaze, which also required 50 homes to be evacuated and closed roads in the area.
Police, who earlier today said the fire appeared to have been deliberately lit, said no one else was being sought in relation to the fire.
The property boasts nearly 300 rooms and was once used as a Catholic seminary. It is understood the hall was unoccupied at the time of the fire.
Firefighters managed to save the chapel - which is still in good condition - from being damaged.
Christchurch Heritage Trust chairwoman Dr Anna Crighton said "decades of neglect" was to blame for the devastating fire.
The building was not being looked after, and was a well-known spot for squatters and parties, she said.
Hobby photographer Vishal Makwana said he was in the building last month and parts of it were still in good condition. He said the church section was well taken care of by a group of volunteers.
Other parts of the mansion were off-limits because of earthquake damage, and some buildings had fallen into disrepair.
Photographs he took at the time show graffiti-covered walls, curling wallpaper, and vines growing across the ceilings and walls.
A person will appear in the Youth Court next week facing charges related to the fire.
