The fish, named Arnie after Australia's recently retired Olympic champion Ariarne Titmus, was first tagged in early 2022 in Mullaroo Creek, about 13 hours' drive west of Sydney, said Arthur Rylah Institute researcher Zeb Tonkin.

Four years old and weighing in at 1.7 kilograms (3.7 pounds) at the time, it "took off" when floods hit the area in the spring of that year, he said, travelling an initial 760 kilometres upstream in less than two months.

The scaly river dweller, an apex water predator, was able to cover longer distances because barriers had been removed along the Murray River to let flood waters pass through.

"It basically provided free passage for fish," said Tonkin.

Arnie then turned around at some point in the past 12 months, swimming another 100 kilometres downstream towards home.

Researchers only discovered the extent of the fish's travels when sharing data with colleagues a couple of weeks ago.

"We've been working on these species for decades... and we haven't come across that sort of scale of movement beforehand," Tonkin said.

"Probably the best we have seen a Murray cod do is around 160 kilometres."

Understanding Murray cod allows researchers to adapt water flows to protect their environment, said Tonkin, whose institute is part of the state of Victoria's department of energy, environment and climate action.

The fish -- not related to ocean cod species -- can live more than 48 years, grow to about 1.8 metres and weigh more than 83 kilograms (183 pounds), according to the government's survey of Australian fish stocks.

"If we see this species breeding, growing well, migrating well, it's a pretty good indication that the ecosystem in general is pretty healthy," Tonkin said.

The decade-long fish tagging project works closely with the Mallee Catchment Management Authority. It is funded by the state-financed Living Murray program, managed by the Murray-Darling Basin Authority.