Environment Southland staff say they are "concerned" as flood water threatens an old paper mill storing chemicals which could turn toxic when mixed with water.
Thousands of tonnes of a potentially dangerous chemical — known as ouvea premix — are still stored in the former Carter Holt Harvey paper mill in Mataura.
The dross from the New Zealand Aluminium Smelters' (NZAS) Tiwai Point smelter is classified as a class 6 hazardous substance, which can generate poisonous ammonia gas if it comes into contact with water.
Emergency Management Southland said emergency services and others have carried out further flood protection works by sandbagging around the building.