Cigarettes and cash were taken during the robbery from Elles Rd Night' N Day, at 9.35pm.
Police were keen to talk to anyone who was near the dairy around the time the robbery occurred, or who may have been offered cigarettes, or who knows someone with an unexplained amount of cash, Detective Sergeant John Kean, of Invercargill, said.
Police were confident with the public's help that the offender would be caught and were working with dairies, service stations and other retailers to provide crime prevention advice.
Anyone with information was asked to contact Invercargill police on (03) 211-0400, or anonymously via Crimestoppers on 0800 555-111.