Almost 2000 dead fish and eels were found in Kaputone Creek in Belfast, north Christchurch, in April.

A cold storage company has admitted leaking poison into a north Christchurch stream where almost 2000 eels were found dead.

Polarcold Stores Ltd broke strict environmental laws when it released ammonia into the waterway in April.

The fish were discovered in a four-kilometre stretch of Kaputone Creek in Belfast.

Eels, some as small as 15cm and others up to a metre in length, made up the bulk of the dead animals, along with bullies and whitebait.

The eels were a mixture of shortfins and longfins – the latter a rare species that is declining in number and regarded as being at risk.

GEORGE HEARD/STUFF Many of the eels were longfins, a rare species that is declining in number.

Stuff found dead fish in a section of the stream between a Silver Fern Farms plant – which was not involved – and where the waterway flows into the Styx River at Marshland Rd.

Polarcold Stores admitted in the Christchurch District Court on Wednesday a single charge in breach of the Resource Management Act.

A second charge, relating to a discharge of a contaminant to air, is yet to be heard by the court.

The same two charges will be brought against Russell Trent, an engineering manager at Polarcold Stores' Timaru branch, at a hearing next month.

The company could face a fine of up to $600,000 for its part in the incident. Trent, if guilty, could face up to two years in prison and a fine of up to $300,000.

The spillage happened on the site of an old freezing works on Belfast Rd, a few hundred metres from Kaputone Creek.

Polarcold Stores had leased the site and was in the process of preparing for the decommissioning of the site when the spillage occurred.

Ammonia is used widely in the refrigeration industry, particularly for large-scale cold storage of food and in commercial facilities.

GEORGE HEARD/STUFF Storage company Polarcold Stores has admitted one charge over the eels' deaths. An individual will be also appear in court next month.

But its uncontrolled release can cause serious health and safety risks, potentially causing severe permanent disabilities, life-shortening illnesses or death.

In the year to May, approximately 20 ammonia releases were notified to WorkSafe New Zealand, incidents which led to some workers being hospitalised.

Polarcold Stores operates cool and cold stores across Dunedin, Timaru and Christchurch, while a sister company operates on the North Island.

Owned by Christchurch-based Scales Corporation, a sale was agreed in May to the multinational company Emergent Cold for $151 million but is still awaiting government approval.