Taranaki Regional Council have opened an investigation after "thousands" of dead eels were found in the stream running next to Silver Fern Farms' Hawera plant following an ammonia leak last week.

Dead eels in the Tawhiti Stream near the Silver Fern Farms meatworks in Hawera after an ammonia leak. Source: Supplied

Employees of the meat processing plant were evacuated on Wednesday afternoon after a valve failure led to a significant leak of ammonia.

A one-kilometre cordon was set up and the leak was controlled, and the plant was handed back to Silver Ferns Farms by emergency services about 1am on Thursday.

In a statement this morning, Taranaki Regional Council (TRC) confirmed they were alerted to reports of dead fish - mostly longfin and shortfin eels - in the nearby Tawhiti Stream on Friday afternoon.

They said in the statement that the dead fish were "likely linked to an earlier emergency at the upstream Silver Ferns Farms plant.

Silver Fern Farms' Hawera meat processing plant. Source: Google Streetview

"Investigation showed this was a significant event, with more than 1000 fish (mainly eels) dead or dying," the statement reads.

"The Council immediately liaised with Iwi and South Taranaki District Council - the STDC has erected public health warning signs that will stay in place for as long as needed."

The council said heavy rain on Friday had likely flushed out the stream by now, but that the impacts on the Tāngāhoe River are still being assessed.

Local iwi Ngāti Ruanui has placed a rāhui on the stream and warning signs are in place.

TRC said in their statement that Silver Fern Farms is co-operating fully with the investigation.

SILVER FERN FARMS 'DEEPLY DISAPPOINTED'

Silver Fern Farms' head of communications and sustainability Justin Courtney confirmed that "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," Mr Courtney said.

"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."

'DEVASTATING' EFFECT ON IWI

Ngāti Ruanui chief executive and Māori Party candidate Debbie Ngarewa-Packer says her iwi was notified on Friday night of the dead eels, and immediately put a rāhui on the stream and had signs put up to protect locals.

Debbie Ngarewa-Packer Source: Supplied

Ms Ngarewa-Packer said her iwi was "devastated" by the loss of the tuna (freshwater eels), both as a source of food and as an indicator of the health of their waterway.

"The reality is - it's our whole catchment," she said.

"We've been out every day since we heard on Friday, and a lot have been washed out and a lot have been collected.

"It's taken 20 plus years to get them re-populated and gets those streams strong."

Ms Ngarewa-Packer said a local kaumatua had raised the issue of relying on eels as an indicator of the waterway's health.

"As soon as something's wrong in the water, they'll get sick, they'll give us a sign that things aren't good in a particular patch," she said.

"We've now got no kaitiaki to let us know the state of our wai."

A file image of a New Zealand longfin eel in the Waitakere ranges. Source: David Burgess/Wikimedia Commons

There had been a lot of grief over the weekend, she said, as well as anger.

Ms Ngarewa-Packer had discussed "taking Silver Ferns Farm to task in terms of being part of the remedial plan to get the stream repopulated.

"We met with them on Saturday - we had a lot of upset people in the room," she said.

"It wasn't a deliberate release - I guess they had processes that should have kicked into place, and they fell short.

"None of us are quite clear on how, but it did go into the stormwater - it just seems that it escalated quite quickly, and that was the consequence.

"I think maybe Silver Fern has probably got some major challenges and questions to answer."