Bank staff in Christchurch were threatened with a firearm in an armed robbery in Christchurch this afternoon, police say.

The incident occurred at KiwiBank on Riccarton Rd about 12.30pm.

Staff were threatened with a firearm by a man wearing a white mask, with a dark balaclava on top, police said in a written statement.

After receiving a sum of cash, the man left on a red mountain bike.

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A scene examination, including the gathering of CCTV footage, is underway.

Bank staff and other witnesses are being interviewed, with Victim Support helping those affected.

Sightings of the alleged offender, described as a middle-aged Caucasian wearing a silver helmet, faded jeans, a black zip-up style polo shirt with a green T-shirt underneath and grey gloves, should be reported to Detective Senior Sergeant Leairne Dow at Christchurch Police on (03) 363 7400.

The man was also carrying a black bag and a long, thin firearm, police said.