Fire and Emergency New Zealand staff are understood to be dealing with an incident at a facility in Mataura which stores potentially poisonous ouvea premix.
The substance - a byproduct of production at New Zealand Aluminium Smelters' Tiwai Point facility - can create a toxic ammonia gas cloud when mixed with water.
A Fenz spokeswoman said a sprinkler had been activated after appearing to have broken because of frost.
Mataura firefighters were investigating further this afternoon and a hazmat unit from Invercargill was also responding.
State Highway 93 has been closed between Clinton and Mataura.