About 20 fish have died in mysterious circumstances in the Awahou Stream near Windsor Park in Hastings.
Hawke's Bay Regional Council group manager policy and regulation Katrina Brunton confirmed council had been notified of the dead fish on Wednesday.
About 20 dead perch and goldfish, measuring about 15cm-long were found floating in the pond area - no native fish or eels were found dead.
"There were many other fish in the pond alive and healthy."
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Burton said water quality testing showed the water to be clear, with neutral acidity levels and a regular temperature.
There was no sign of any toxins or poison.
"At this stage the cause of death of the fish is not known."
Perch are an introduced pest species in New Zealand, and the council did not know when they were put in the pond, Brunton said.
Anyone with information is asked to call the regional council's 24/7 pollution hotline 0800 108 838.