Dumped pet goldfish have been found in Australian estuaries, prompting fears the fish could make their way into connected river systems and kill off native species.

Last year, scientists from the same university discovered giant, football-sized goldfish weighing more than two kilograms stalking the waterways.

Now new research has revealed the invasive species are living in salt water, which means they can travel between river systems and colonise more ecosystems.

Murdoch University's Dr James Tweedley and team discovered the goldfish while conducting fish surveys in Vasse and Wonnerup estuaries, in Western Australia's South West.

One of the 526 salt-resistant goldfish found in the Vasse estuary by a team from Murdoch University. ( Supplied: Mark Allen )

He said he was surprised one day in 2012 that his team started pulling up goldfish in their nets.

The academic has now analysed the research from that expedition, and tests measuring the salinity-resilience of the species revealed surprising results.

"Typically goldfish are a species that live in freshwater, like you get from your tap, but we were finding them in an estuary that has salty water," he said.

"To be an introduced species, you really have to be able to adapt to an environment you're thrown in, and goldfish have colonised the world. They're native from China but they live in every continent except for Antarctica.

"It's their plastic biological traits — that they can adapt to anything — that makes them so successful."

Dr James Tweedley is urging people to dispose of unwanted goldfish responsibly and humanely. ( ABC News: Rebecca Trigger )

He said tests revealed that rather than adapting to the salt levels in the estuary, goldfish may always have had some natural resistance to salinity.

"We tested the salinity tolerance of the goldfish from the Vasse estuary, and compared them to some goldfish we purchased from a pet store, and we found they were the same," he said.

Dr Tweedley said the fish owners were releasing them into rivers in the belief it was the kindest way to dispose of unwanted pets, but once released they posed a threat to native species and the ecosystem of the rivers.

"They have a big mouth, and what they do is they feed on the bottom of the water, they scoop the sediment up, and as that sediment comes up it brings with it nutrients, which can fuel algal blooms.

"They are also carnivorous, and they predate on the eggs of our small native fish species, and some of our frogs too."

He is urging people to either return their unwanted pets to the store, give them to a friend, or dispose of them humanely by placing them in very cold water and then putting them in the freezer, which slows their metabolism and eventually causes death.