Polarcold Stores Ltd admitted discharging ammonia on to land in circumstances that could see it enter water, without permission or consent, in court on Wednesday.
Eels, some as small as 15 centimetres and others up to 1 metre in length, made up the bulk of the dead animals, along with bullies and whitebait, Stuff reports.
This included a rare longfin species and 30 fish.
In April this year the ammonia was found in a 4km stretch of the Kaputone Creek in the suburb of Belfast, along with the dead eels and fish.
After an investigation lasting several months, the regional council laid criminal charges against Polarcold.
The company faced a maximum $600,000 fine.
It had yet to enter a plea on a second charge of releasing ammonia gas in to the air.
An individual also faced two charges of illegally discharging the chemical.
They will next appear in court on 19 December along with the company.
- additional reporting RNZ
