More than 100 dead eels were discovered in the Ruahapia Stream along Kenilworth Road in Hastings last week, with the Hawke's Bay Regional Council (HBRC) reporting that a pollution incident in the stream had caused the deaths.
The HBRC has been working closely with the Hastings District Council to investigate, trying to get a complete picture of what happened.
The councils are waiting until they better understand the eels' cause of death to inform the next steps they will take.
Hawke's Bay Department of Conversation operations manager Tryphena Cracknell said this incident is a "very serious situation".
"Ancestors of New Zealand's tuna (eels) were swimming our waters at least 23 million years ago and their modern-day descendants are struggling for habitat due to historic[al] wetland removal, commercial land and water use, and historical commercial fishing practices," Cracknell said.
DOC has no further information about the incident as the situation is being managed by the HBRC.
However, Cracknell said: "We hope that the investigation can find out what happened and preventive measures put in place to support clean, safe habitat for eels and other freshwater fish."
HBRC's group manager of regulation Katrina Brunton said the teams are following up on some "positive lines of enquiry".
The manager said that environmental pollution like the incident in Ruahapia Stream is unacceptable.
"We have zero tolerance for pollution incidents, particularly where they cause significant environmental harm."
The councils are appealing to members of the public who may have any information to contact the 24/7 Pollution Hotline on 0800 108 838, which can be done anonymously.
A HBRC spokeswoman said the team should have more information on the Ruahapia Stream pollution incident and the eels' death at the end of the week or early next week.