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.