Fire investigators are determining whether a fire in Christchurch's Cathedral Square is suspicious.

Six crews were called to a building next to the old BNZ building just after midday where a fire had broken out in the ceiling.

Fire spokesman Brett Dunn said it appeared a couch was set on fire but at this stage they don't know how.

Fire services were alerted after receiving multiple calls from members of the public.

Dunn said streets were cordoned off as firefighters put out the blaze.

He said no one was in the building at the time and crews managed to extinguish the blaze before any major damage was done.

It's unknown whether it's a vacant building though the Canterbury Earthquake Recovery Authority was believed to be onsite.