Twenty years of work to replenish eel stocks in a South Taranaki stream was undone by a chemical spill from a meat processing works, a "grief-stricken" Taranaki iwi says.

Officially, more than 1000 fish, mostly eels, died after ammonia spilled from the Silver Fern Farms site in Hāwera into the Tawhiti Stream on Wednesday, February 19, but local iwi say the death toll is much higher.

A gas leak from a broken valve had been reported that afternoon, leading to emergency services putting a one-kilometre cordon around the site for several hours before an all-clear was declared. Two people were also treated in hospital.

But two days later the Taranaki Regional Council was alerted to the Tawhiti Stream, which runs alongside the Silver Ferns site on Tawhiti Rd, where eels were found dead or dying.

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Debbie Ngarewa-Packer, chief executive of local iwi Ngāti Ruanui, said they were "gutted" because eels are known as a kaitiaki (guardian) in Māori culture.

"There has been a complete loss of life," Ngarewa-Packer said.

"We have been told there will be nothing left."

Silver Fern Farms has taken responsibility for the mass death and the Taranaki Regional Council is investigating.

ANDY JACKSON/STUFF The meat works was evacuated on Wednesday, February 19, after the ammonia spill, which hospitalised two.

Ammonia is used in fertiliser production, air conditioning, refrigeration, manufacturing and is also an ingredient in many cleaning products.

At room temperature, ammonia is a colourless gas with a suffocating odour, and it forms a clear liquid under pressure.

High concentrations in the air can irritate the skin, eyes and throat and cause burns.

Ngarewa-Packer said she understood the leaking ammonia had gone down the stormwater drain, rather than into a separated tank, and ran straight into the stream.

Although the regional council said more than 1000 fish and eels had died, Ngarewa-Packer said she had been told the death toll was in the thousands.

"I said, 'Have I heard right?'"

Ngāti Ruanui had spent the last two decades replenishing the eel population, which included the longfin species, in the stream.

Eels, known as tuna in te reo, are special in Māori culture as their absence can be a sign that water is polluted.

"It's so sad they had to die for us to know it's unsafe."

ANDY JACKSON/STUFF Ngapari Nui, Turangapito Parata, and Debbie Ngarewa-Packer have been actively checking the stream and river in hopes they will soon return to normal.

Ngarewa-Packer said Ngāti Ruanui expected Silver Fern Farms to work to replenish the eel population.

"They will be held accountable by us."

The iwi has placed a rāhui on the Tawhiti Stream and the Tangahoe River, into which it runs before going out to sea, until further notice.

This means no one can collect food from either waterway and the South Taranaki District Council (STDC) has warned against swimming in them.

In an emailed statement on Monday, Fred McLay, Taranaki Regional Council's director of resource management, said it was believed the death of the eels was "likely linked" to the ammonia leak.

"Investigation showed this was a significant event," McLay's statement read.

McLay said heavy rain on Friday night had flushed the stream, and any impacts on the river were being assessed.

"Silver Fern Farms is fully cooperating with the investigation and dealing with public inquiries.

"The Taranaki Regional Council's investigation is continuing."

The council said it was unlikely to make any further comment while the investigation takes place and no member of staff would be available for interview.

ANDY JACKSON/STUFF The iwi has put a rāhui on the Tawhiti Stream and Tangahoe River, and the South Taranaki District Council has warned against swimming in them too.

Justin Courtney, Silver Ferns' head of communications and sustainability, did not answer phone calls on Monday but did release an emailed statement saying: "We believe the ammonia entered the stream during the emergency and fish have died.

"We are deeply disappointed this has happened given our commitment to our environment and the stream restoration work we had carried out on the Tawhiti Stream.

"Our focus is on the clean-up and to ensure no more harm is caused.

"We take our responsibility around our environment and our engagement with the local community seriously and have met with Ngāti Ruanui to work with them to put this right.

"We are committed to a full investigation, but it is too early to know the cause for how the ammonia got into the stream. We will make the results of that investigation public."