A "large number" of dead eels was reported by recreational kayakers to the Department of Conservation (Doc), who advised the council in May.
The incident, including the council's investigation, was discussed in reports to the Resource Management Committee on July 9.
Council compliance manager Chris Barnes told Local Democracy Reporting the matter follows a similar incident on the Arnold River in 2022.
"It's an ongoing investigation. We have had a meeting with Doc, iwi and Manawa (Energy). Nothing has really come of it yet," he said.
However, council was making a number of formal information requests to get to the bottom of it under a formal investigation.
Mr Barnes noted that members of the public reporting dead eels in the same location had occurred a few times in the past decade.
Fish screens fitted at the Arnold Dam were supposed to alleviate the risk to species like eels, he said.
"There's a lot that needs to be looked at … this time we're not going to let it lie."
The latest incident almost mirrored exactly one in early 2022 when recreational kayakers contacted Doc about dead eels floating in the river.
At the time, a council investigation found the required fish screens under the power scheme's consent were in place.
In a statement, Manawa Energy general manager generation Todd Mead said they were working with the regional council and other key stakeholders to investigate eel deaths downstream of the Arnold Dam.
"The outcome of this investigation is expected over the next couple of months," Mr Mead said.
At the time of the eel mortality event in May, Manawa Energy was operating the Arnold hydro scheme in accordance with resource consents and statutory obligations and continued to do so.
"Manawa Energy also continues to operate the successful upstream trap and transfer programme at Arnold Dam, which was established in the early 2000s."
- By Brendon McMahon
Local democracy reporter
