As Southland continues to be inundated with rain and floods ravage the area, the threat of a rising river poses potential risks to locals.
A toxic ammonia gas could be released if water from the nearly-overflowing Mataura River seeps into an old paper mill.
The paper mill houses ouvea - a substance that, when mixed with water, creates the toxic gas.
However, Civil Defence believe the mill is well protected.
Ouvea is a premix comprising of about 30 percent aluminium oxide, 30 percent aluminium nitride and 30 percent magnesium aluminate, with small amounts of metallic aluminium.
But how potentially dangerous is the threat of ammonia? The Science Media Centre gathered comments from two scientists to assess the situation.
Associate Professor Sally Gaw and Professor Brett Robinson, School of Physical and Chemical Sciences at the University of Canterbury
Associate Professor Sally Gaw and Professor Brett Robinson say if the storage site is flooded, the surrounding area will need to be evacuated and all potentially exposed people will need medical attention.
This is because ammonia gas can damage the skin, lungs and eyes, and in severe cases can cause death.
And if ammonia is produced, the Mataura River could feel the effects.
"Ammonia gas dissolves in water to produce ammonium which is toxic to people, aquatic organisms and stock and depending on the amount of ouvea coming in contact with the floodwaters, toxic effects could occur downstream as far as the estuary," they say.
Overtime, the ammonium converts to nitrate and can trigger the growth of algal blooms - which is also toxic to aquatic organisms.
They believe the worst case scenario is the heat released by the reaction with water, which could generate a fire at the paper mill along with releasing ammonia gas.
"The actual impacts will depend on how much of the ouvea comes into contact with water."
Professor Allan Blackman, School of Science at Auckland University of Technology
Of the ouvea premix materials, Professor Allan Blackman says the "most potentially hazardous" compound is aluminium nitride, which is what makes ammonia when mixed with water.
He also has concerns about heat produced from the reaction when ammonia is made. This is because heat aids in the formation of hydrogen gas and could cause a potential explosion. But this will only happen if "large enough quantities" of hydrogen are generated.
But the reaction time of aluminium nitride with water is relatively slow, he says. Which means the release rate of ammonia would be similarly slow, as would the production of hydrogen.
Angus McKay, Southland Civil Defence controller
The threat of ammonia led to Angus McKay releasing a statement to "quell speculation and misinformation" about the ouvea premix.
He says sandbags are stacked at the site and there is a large cordon around the paper mill due to 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.
He predicts that because there's so much water flowing into the river, any contaminants should be diluted.
"The area is still evacuated. We have had no reports of any ammonia coming from the paper mill."
You can follow along with Newshub's live updates on the Southland floods here.