Emergency services were called to BOC Gas and Gear in Orari St, following reports of a leaking gas bottle just after 1pm.
Three fire appliances and a fire command unit were at the scene, and police blocked both ends of the street. They were soon joined by dozens of evacuated workers.
Dunedin station officer Phillip De Rooy said the gas was hydrogen sulphide, a poisonous and flammable gas which required firefighters to wear appropriate equipment and breathing apparatus.
Firefighters were able to isolate the source of the leak, which was from a small gas cylinder, and the cordon was lifted just before 3pm.