The ACT's Environment and Planning Directorate received a report of 19 dead fish. The Environment Protection Authority, a fish ecologist and government vet attended to find Murray cod and a golden perch.

Water measurement and fish samples were taken for further investigation. No results have been released.

Adam Samios, who provides regular fishing reports for the media, says he was contacted by a Canberra fisherman on Tuesday night who saw between 12 and 15 dead Murray cod between 45cm and 50cm long near the western bank.

The fish were floating in the water, near an area where 48 reef balls - hollow concrete domes of varying sizes - were lowered into the pond in January, 2013, for additional habitat for native fish.

Mr Samios said the dead fish were reported to the Environment Protection Authority. . "They (the agency) were out there on Saturday and had found some and were aware of it, which is a bit upsetting," Mr Samios said.

He said the angler had been fishing for golden perch when he found the dead fish all within a short distance of one another.

Last year a large number of golden perch were found dead on the pond's island.

"I believe that was people catching them, rather than an actual fish kill. We do get the EHN (Epizootic Haematopoietic Necrosis ) virus which affects redfin each year, I haven't come across anything like this with cod."

Canberra ecologist and long-time fishing commentator Bryan Pratt said the fish could have died from being mishandled, or from spawning stress.

"It is not uncommon for fish to die during spawning periods. But 12 is a lot of fish to die, so there might be something else going on.

"In a small lake like that, if someone can find 12 dead ones fairly quickly, that means there could be a lot more floating around dead along the shore line.

"I've been getting phone calls all day ( Wednesday) from kayakers, walkers, anglers, bird watchers and others about dead Murray cod in Yerrabi.

"It now seems as though quite a large number of fish may have died, given that reports of dead fish have come from all parts of the lake. Most fish seem to be in the 50cm to 70cm range."

Yerrabi pond is a favourite of anglers because of the scarcity of carp.