Environment and Planning Directorate aquatic ecologist Dr Lisa Evans said the ACT Government had been doing testing to establish why fish in the pond had been dying.

Large and small cod were found dead at Yerrabi and Gungahlin ponds earlier this month.

Fishing enthusiasts have been concerned the entire large cod population would be wiped out, which would be a setback after years of stocking.

Dr Evans said tests on water samples all returned readings within normal ranges.

Results from tests done by the University of Sydney have found fish viruses are not to blame for the deaths.

Dr Evans said it was believed the cause might be linked to short-term oxygen shortage associated with nutrient run from recent storms, warmer water temperatures and large amounts of filamentous algae in the pond.

She said breeding stress could also be a contributing factor.

"This is the first time that a Murray Cod fish kill has been observed in Yerrabi Pond in the 14 years  the pond has been stocked with these fish," she said.

Dr Evans said the recent deaths were a timely reminder to Canberrans of how fish diseases could be transferred by people dumping unwanted pet fish into waterways and illegally moving fish between waterways.

Dissolved oxygen levels in the pond water have since returned to normal.

Any dead fish in or around local waterways should be reported to Canberra Connect on 13 22 81.