Flood water from the Mataura River has now seeped inside an old paper mill in Mataura, Southland storing thousands of tonnes of a hazardous substance.
The substane - ouvea premix - has been left abandoned in the old paper mill building since 2016.
The gas is also hazardous to the environment.
When saturated with water ouvea premix creates ammonia gas which is toxic to humans.
The Mataura River is expected to peak with 2,740 cumecs at Wyndham at 3.20pm today. This is 4.21 metres above normal river level.
1 NEWS understands the flood water has breached inside the century-old Mataura Paper Mills building which lies right on the Mataura River.
Angus Mckay of Southland Civil Defence says there is so much water going down the river that any contaminants that leach into it will be diluted.
He says there is also "a very large cordon" around the paper mill due to the flooding.
“We are aware of the potential risks of the ouvea premix. We’ve got a wide evacuation zone around the area. The risks associated with the premix have been considered when setting the evacuation zones around the paper mill," said Mr Mckay.
“The area is still evacuated. We have had no reports of any ammonia coming from the paper mill.”
Emergency services and others have carried out further flood protection works this morning by sandbagging around the building.
Emergency Management Southland said it is co-ordinating with other relevant agencies, including iwi.
Severe weather in Southland has caused the Mataura River to rise significantly, prompting evacuations of low-lying areas at Gore, Mataura and Wyndham.