Fire and Emergency New Zealand southern communications shift manager Simon Lyford said crews were alerted via a fire alarm at South Pacific Meats in Awarua, Invercargill, about noon.
Two appliances were initially sent from Invercargill and the plant evacuated.
"They responded to a possible ammonia leak possibly due to a power cut."
A second alarm was then transmitted once it was a confirmed to be a leak and a HAZMAT team was dispatched.
Cordons were established and the leak had been isolated by about 1.30pm.
About 10 minutes later, the job had been handed back to the company owner.
No injuries were reported and it was understood staff had returned to the building to resume work.