Mao Zhou (39) was sitting behind the counter looking at the internet when he heard the door of his Halfway Bush dairy open at 8.30pm on Sunday.
Standing to greet the customer, he was confronted by a balaclava-wearing man who demanded "Kronic" as he began to pull out a firearm.
Mr Zhou said instinct took over and he ran to another room for safety, locking an inside door to the shop behind him.
"It just happened all so fast.
"I had no time to think about it."
Mr Zhou said a later stocktake revealed the armed assailant had run off with $100 worth of Kronic - a "legal high" billed as a substitute to marijuana.
"I am not going to sell this anymore ... it is not worth it."
Detective Graeme Smaill said police were concerned, not with the product that was taken but "the action that was taken".
The shopkeeper's evasive action was a "fairly wise move", given the man was armed.
"He got himself out, he let the guy do what he wanted to do and it turned out as well as could be expected."
Det Smaill confirmed police were working on several leads, including witnesses providing a description of the robber and of his car.
Witnesses saw the man remove his balaclava as he entered his car outside the dairy, on the corner of Taieri Rd and Ashmore St.
He was described as a Pakeha, aged between 25 and 35, with short cropped brown hair, a 10cm long goatee, and wearing a dark hoodie with white writing.
The man drove off in a blue/green Subaru Legacy, with a partial licence plate sighting of "EAU", but police were keeping an "open mind" on this reading, he said.
He drove down Balmain St, before "looping back" on Taieri Rd at high speed, past the dairy and in the direction of Three Mile Hill Rd.
Det Smaill said the firearm was described as an older type, between 50cm and 60cm in length, with a wooden stock and dark barrel.
Police did not believe other stores selling Kronic would be targeted by the man.
Anyone with information was urged to contact Dunedin police on 03 471-4800, and ask for the team working on Operation Ashmore.
