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

Environment Canterbury (ECan) is investigating the source of a contamination in a north Christchurch stream that left almost 1000 eels and fish dead.

Hundreds of lifeless fish were discovered in a stretch of Kaputone Creek in Belfast on Wednesday.

Eels made up the bulk of the dead animals, some as small as 15 centimetres, others up to 1 metre in length.

GEORGE HEARD/STUFF The source of the contamination is not yet known.

Stuff found dead fish in a section of the creek between Silver Fern Farms' Belfast plant and where it flows into the Styx River at Marshland Rd.

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Eels were often clumped together in groups on the stream bed, while some were washed up at the banks.

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

It is not yet clear what species of fish were killed, and no dead birds or other animals were spotted.

Silver Fern Farms plant manager Phil Kilgour told Stuff he was unaware of the dead fish and had not been contacted by ECan.

He and his staff later went out to take water samples along the stream. There is no evidence to suggest the contamination came from the meat processor's site.

GEORGE HEARD/STUFF The stream is close to two water bottling plants and a meat processing plant.

The section of the creek is also close to the site of two controversial water bottling plants that have consent to take billions of litres of water from beneath Christchurch for a minimal cost.

ECan staff were alerted to the dead fish on Wednesday morning and four investigators spent the day checking the stream, testing water and assessing downstream, as well as talking to local businesses.

Steve Firth, ECan's local acting zone manager, said: "Several staff went to the site and found about 800 dead eels and fish along a four-kilometre stretch.

"Along this stretch the contaminant has diluted due to the good water flow which means that after this point we found schools of live fish and eels.

"It's possible the contaminant came through the storm water network, but it's also possible that someone dumped something into the stream.

"At this stage, we don't know what the contaminant was."

Firth said investigators believe the contaminant entered the water some time between 4pm on Tuesday and 7am on Wednesday.

"They believe whatever contaminant entered the water, it was swift acting, but diluted quickly. We'll have staff visiting the area [on Thursday]."

Results of the water samples are expected to take at least 48 hours.

Canterbury medical officer of health Dr Ramon Pink advised people to stay out of the water and keep pets away from the stream.

Anyone who has been in the water or who has related health concerns should contact their doctor.

Members of the public with information about the contamination should call ECan on 0800 765 588.