Claims that the Murray cod that perished in a mass fish death in the Darling River this week had lived through "two world wars" and were a century old have been disputed by an academic.

As emotions run high in Menindee, near Broken Hill, NSW Opposition Leader Michael Daley yesterday tweeted the iconic fish had survived the great depression before dying in a blue green algal bloom.

Loading

Claims the cod were 100 years old also hit the spotlight in a viral video this week, in which two emotional farmers lamented the fact that up to a million fish had died in the river.

The video has had more than 4 million views on Facebook.

Though no one contests this week's incident is a tragedy, some have questioned the claim made by one of the farmers, Rob McBride.

Matthew O'Connell is finishing his PhD studying taxidermised Murray cod and said the existence of a century-old fish was unlikely.

"The oldest one on record that's been looked at [is] 48 years old," he said.

"They're a very long lived fish, it wouldn't be unusual for a fish to be 30 to 40 years old.

"But a 100-year-old fish would be an exceptional fish, and there's been nothing like that ever recorded in the literature, in the science."

Dead fish float on the Darling River as far as the eye can see. ( Twitter: Gary Robert Looney )

Speaking to the ABC, Mr McBride said local fisherman were certain some of the cod were between 80 to 100 years old.

"If I got the dates wrong I'm sorry," he said. "I'm a farmer not a fisherman."

He said people focusing on the age of the fish needed to "get a life".

"Two men in their 50s are holding two monsters in a dead river system that are dead themselves, if they gloss over that issue then they've really lost the whole point," he said.

"This isn't a short-term issue, this is long-term, man-made destruction of the river system and I guess that's the key message we hope people take away."

The Murray cod is the biggest freshwater fish in Australia and has the longest lifespan.

Despite their longevity many of these fish fell victim to an oxygen eating blue green algal bloom, the second to strike Menindee in a month.

Premier Gladys Berejiklian called the event a "tragedy" and blamed the severe drought gripping the state.

The NSW Government has asked the Department of Primary Industries and WaterNSW for a report on the fish kill and clean-up process.