"I'm five feet, four inches (162 centimetres), so you can get an idea of the size of the fish," said Mr Starr, who found the fish on Monday, among more than 60 which have died since last week.

"I'd always dreamt of catching and releasing a cod this size and was so saddened to see them lying dead around the . . . Yerrabi Pond [in Gungahlin]."

Ecologist Dr Bryan Pratt said Yerrabi, which was first stocked with 5000 cod about 14 years ago, was in danger of losing its population of large cod because either a pathogen or a parasite was targeting the older fish.

"That is a real sad loss given the effort involved in purchasing the fish originally, stocking them and having them grow like that," Dr Pratt said.

"They eat vast quantities of redfin and carp," Dr Pratt said. "The lake has only recently had carp in it. Carp haven't taken over in the same proportion as they have in the other urban lakes.

"Given the numbers we have seen floating dead around the shoreline, there is likely to be many multiple times that dying, where they are not seen. We could be looking at the tip of the iceberg.

"It may not be all that dramatic for the general public, but for the angling and ecological world it is very concerning, given all the things we have done to protect the lake environment, to suddenly have it implode in this fashion is quite significant.

"Fish are at the apex of the system; locally, Murray cod are at the apex of those fish so when the top of the chain collapses, there is something terribly wrong going on."

Brett, an angler who declined to give his surname, said his seven-year-old son was in tears at the sight of so many dead cod.

'I've been fishing there with my little boy for five or six years. It's where he caught his first fish," he said.

The angler had measured dead cod which included 65-centimetre, 75-centimetre, 82-centimetre, 94-centimetre and 96-centimetre fish.

"They are significantly big fish. One minute they are big, fat healthy fish, now they are all belly up."

He said fishermen released cod to enable them to flourish and would be devastated by what was happening at Yerrabi.

An ACT Environment and Planning directorate spokeswoman dismissed suggestions the deaths could be related to work beginning upstream in the new suburb of Moncrieff.

"The clarity and sediment load in Yerrabi Pond is still much better than waterways of the Murray-Darling system where large natural populations of cod exist," the spokeswoman said.

Analysis of water samples taken last week were not yet available. However, more samples have been taken.