Environment Southland said yesterday its compliance and science teams investigated after a report of dead eels in the Low Burn Stream.
Compliance manager Donna Ferguson said the team estimated there were several thousand dead eels over about a 10km distance.
"Our staff have spent extensive time out investigating any potential causes and we have identified one potential source.
"We have sent a suite of samples away for testing and will continue to look for further evidence."
It was likely people would continue to see dead eels as they made their way down the waterway and into the Mataura River.
"We know the deaths of these eels and other stream life in the area will be distressing.
"This is an active investigation and our teams are working hard to establish the cause."
People were asked to avoid contact with the water.
- Further eel deaths: Call 0800 76 88 45.
