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Alexandra Volunteer Fire Brigade Chief Fire Officer Russell Anderson said contractors working at the Hinton Estate coolstore in Chicago St cut into a pipe they believed had been isolated.
That sparked an emergency services callout that resulted in a 100m cordon being placed around the coolstore.
Central Otago sub area commander Senior Sergeant Ian Kerrisk said from the six houses within the evacuation zone, two people were evacuated to outside the cordon. People at businesses and Dunstan Educare, an early childhood centre, were told to stay indoors as the situation was treated as potentially explosive.
Although it took the brigade about two hours to bring the situation under control, it took them less than 15 minutes to isolate the system and turn off the gas valve, Mr Anderson said.
''It all takes time; it's not something you can just rock up to and do.''
He said the contractors were upgrading safety valves on the refrigeration unit but when they cut through the pipes, they found the system was not isolated as they had been led by another contractor to believe. He said the contractor who cut the pipe was taken to Dunstan Hospital as a precaution.
''They [the contractors] reacted pretty quickly; there were a couple of breathing suits in that room.''
He said the contractors were the only people working at the coolstore, as the fruit-packing season was yet to begin.
A Ministry of Business, Innovation and Employment spokeswoman said it was making ''preliminary inquiries''.
Mr Anderson said the Otago Regional Council, the Central Otago District Council and the Southern District Health Board had been notified, as had a hazardous substances technical liaison officer.
He said any residual ammonia would dissipate into the atmosphere.
Howie Hinton said the incident would not affect their business.