Several thousand dead eels have been found in a stream near Mataura, in Southland.
Environment Southland's compliance and science teams have launched an investigation after receiving a report of dead eels last week.
Compliance manager Donna Ferguson said the eels were found over a 10km span of the Low Burn Stream.
"Our staff have spent extensive time out investigating any potential causes and we have identified one potential source," Ferguson said.
"We have sent a suite of samples away for testing and will continue to look for further evidence."
The bodies of the eels were likely to make their way down the waterway and into the Mataura River.
People would likely continue to see them as they moved through the waterway, Ferguson said.
"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."
Ferguson asked the public to avoid contact with the water in the stream.
Any further eel deaths could be reported to the compliance team on 0800 76 88 45, she said.