Fire crews are scrambling to isolate a leak of hazardous chemicals at a freezing works near Invercargill.

Fire Service Southern Communications shift manager Riwai Grace said they were called to the ammonia gas leak at 1.04pm on Crowe Rd, Wallacetown.

The incident happened at Alliance's Lorneville plant.

Eight vehicles including fire crews, support vehicles and water tankers were at the scene trying to find the leak.

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"We've denuded Invercargill City of all fire appliances. We've had to bring in Bluff to cover Invercargill," Grace told the Herald.

"Fire crews are trying to find the leak, isolate it and then they'll go from there. At this stage it's not contained.

"[The gas] has the effect to do some serious issues."

Ammonia poisoning can result in symptoms like severe chest pain, difficulty breathing, fever, temporary blindness, throat pain, vomiting, collapse and shock.

Alliance Group spokesman Kerry Stevens said the leak had sprung in the engine room of the refrigeration area.

"At the time of the leak, processing at the plant had already concluded for the day so the majority of the staff had left the site.

"As is standard procedure, the remaining staff were evacuated and no staff were injured.

"The health and safety of our people is our number one priority and we've launched our own inquiry. We will also be co-operating fully with the Fire Service's investigation."

A watch house operator said workers were in good spirits. He said there was a major power cut, with an ammonia leak, and workers were sitting around in the building trying to keep warm.

An ambulance is in attendance.