Eighty fish have washed up dead in south-west Queensland and hundreds of crabs have also perished on a south-east island amid extreme conditions.

It is estimated 77 Murray cod and three golden perch have died since Saturday morning in the Ward River, near Charleville.

The local Fishing and Restocking Club president believed the deaths were caused by a "black water event" after years of drought followed by heavy rainfall.

"Dirty water has washed down and took all the oxygen out of our river and it's killed a lot of cod," he said.

"All the stagnant water sits in the water holes with all the leaf matter, when you get a bit of a run [in the river] it just takes the oxygen out of the water.

"It's the bigger fish, the ones that use the most oxygen, that die.

"I've never seen it before and it kind of shocked my system."

It is believed the deaths were caused by a "black water event" after years of drought followed by heavy rainfall. ( Supplied: Charleville Fishing and Restocking Club )

Mr Ward said the fish deaths occurred at a popular fishing spot, which was devastating for tourists, however those caught in the river can still be eaten.

While the fish deaths seem to have settled down, Mr Ward said more were possible.

"Hopefully everything will go alright and there will be plenty of fish for the tourists and everyone to catch," Mr Ward said.

"We've got a bit of a run now so hopefully it's washed all that black water away, but you never know."

The bigger fish in the river, ones that use more oxygen, died. ( Supplied: Charleville Fishing and Restocking Club )

The fishing club has been restocking the local rivers since 1992.

Since then, they have added 51,300 cod and 604,000 yellow belly in the Warrego and Ward rivers.

The Charleville Fishing and Restocking Club has notified The Department of Environment and Heritage Protection and is waiting for test results from the Murweh Regional Council.

Hundreds of crabs die on Bribie Island

Loading

A mass die off of crabs and yabbies also occurred in the past few days in Mermaid Lagoon on Bribie Island, off Brisbane.

It comes after a five-day heatwave saw temperatures reach more than 35C in and near Brisbane.

The crustaceans lined the lagoon, appearing to have all died around the same time.

"The Department of Environment and Heritage said natural conditions are the likely cause, in particular the recent hot weather and low water levels," a department spokesperson said.

"Marine and freshwater fauna can be adversely impacted when weather conditions result in low dissolved oxygen in the water."