More than 40,000 litres of acid-contaminated water was extracted from a Napier waterway after a hydrochloric acid spill.
The public are still being urged to stay out of the water at the estuary after the spill at Galvanising Hawke's Bay on Friday.
Emergency services were called to the commercial property on Thames St, Pandora, about 4.05pm on February 5.
Four fire trucks, a HazMat unit and support vehicles attended the scene, while Thames St was closed and properties were evacuated.
Napier City Council environmental solutions manager Cameron Burton said the team took the lead on the environmental response in collaboration with Hawke's Bay Regional Council.
The councils provided advice to on-scene fire crews on how to neutralise the acid spill, before council co-ordinated the usage of vacuum trucks which managed to extract 40,000 litres of acid-contaminated water from the upstream side of the spill gate in the Thames waterway throughout the night.
Burton said the team worked into the early hours of Saturday morning carrying out fish-kill assessments in the channel leading from the spill gate onto the mud flats towards Embankment Bridge.
"Findings were distressing for staff who have been working hard to improve water quality in these tributaries of Te Whanganui-a-Orotū (Ahuriri Estuary), through environmental monitoring, rehabilitation planning, the Stormwater Working Group, changes to the Napier City Stormwater Bylaw 2020 and the Stormwater Education Campaign," he said.
The councils urged residents to avoid contact with the estuary's water or mud flats until further notice after the spill entered the stormwater system and the Tyne St drain, which flows into the estuary.
Hawke's Bay District Health Board medical officer of health Dr Nick Jones said anyone suffering related health-effects should contact Healthline on 0800 358 5453 or see their GP.
Fire and Emergency New Zealand said the investigation into the spill was ongoing at this stage.
Napier City Council said staff gathered evidence for the investigation, which will be discussed by HBRC and HBDHB to make a decision on the removal of warning signs.
Burton said contents of NCC's findings, which it will not publicly release, will be used by council's enforcement decisions group to decide whether to press charges.
Galvanising Hawke's Bay has been approached for comment.