An ammonia leak that threatened to force the evacuation of Bluff has been contained.

Senior Station Officer Deane Chalmers, of Invercargill, said a fire in a cool store on a Bluff wharf last night caused a small leak in an ammonia tank and there were fears the situation might escalate.

An 800m exclusion zone was placed around tanks at South Port this afternoon, where the leak was being tended to by the fire service and port services.

Firefighters from Bluff and Invercargill were on scene as was a HazMat unit.

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"As a result of hard work from all agencies involved with this incident, the ammonia leak within the cool store on the Bluff Wharf has been successfully contained," Mr Chalmers said.

"Fire Service staff are now ceasing their operations and will be handing the scene over to the Port Authority, and normal duties will be resuming shortly on the wharf."

South Port chief executive Mark O'Connor said the fire was in a third party tenant's premises, Wilbur-Ellis, a pet food operation.

"Some ammonia product is used in the freezing process,'' Mr O'Connor said.

Fire communications shift manager Tim Reynolds said the fire was in a commercial building and was "well alight'' when two fire appliances arrived from Bluff about 11.30pm.

The fire fire was about 100m long and 40m wide, he said.

There were no reports of injuries, he said.

A fire investigator from Dunedin is on currently on site.