Lunchtime shoppers at a major Levin supermarket on Monday were forced to evacuate the building immediately and leave their groceries in the aisles.
An emergency alarm was raised by a technician working on a refrigeration unit at the time. The gas leak was found shortly after lunchtime.
The alarm was officially raised at 12.12pm and trucks from the Levin Fire and Emergency Service were promptly at the scene, as the Levin station was less than 200m away.
Staff and customers left the building through a designated fire exit and met at an assembly point in the supermarket carpark, where they were assured it was no drill.