Environment Southland is working to determine why thousands of eels continue to be found dead in a stream near Mataura.
Residents have been reporting sightings since last Thursday.
Environment Southland compliance manager Donna Ferguson is estimating several thousand dead eels will be found in the Low Burn waterway within at least 10km.
She said more are being found everyday.
"When we first attended the site on Thursday evening we did notice a large number but it's definitely increased the further we look up the water way."
Environment Southland staff have been working around the clock to determine the problem - speaking to residents, collecting samples and retrieving the eels from the site. They'll be working with the local iwi around disposal.
"The team have spent a considerable amount of time out walking the waterways, speaking to land owners, property owners in the area as well. Just looking for any causation for the deaths that we can find," Ferguson said.
At this stage, Ferguson believes something has contaminated the water as opposed to an illness.
Although Environment Southland has identified one potential source, it may be some time before it can confirm the cause.
"Test results won't be back for another week... but we have sent a suite of samples away for testing and will continue to look for further evidence," said Ferguson.
With stock, factories and a meatworks nearby, the contaminant could be anything, she said.
"There are a number of farms in the area, there's also some industries upstream as well as in the immediate area."
Environment Southland is urging people to avoid the water and to report any further eel deaths to their compliance team on 0800 76 88 45.
