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Detective Sergeant Rob Hanna, of Dunedin, said the image was taken on a security camera at the Green Island Post and Paper shortly before 11am.
The man, who was wearing a black hoodie, sunglasses and jeans, handed over a note to a teller demanding money be put in a bag. The man implied he might have a gun, although none was presented.
Det Sgt Hanna said it was still too early to say whether the man was the same who held up a BP service station in Mornington at 8pm on Sunday.
Images from that robbery could not be released, he said.
Anyone who could assist police with their investigation should contact Dunedin police on 471-4800, or provide anonymous information to the Crime Stoppers line on 0800-555-111.