The Department of Fisheries have confirmed around 30,000 fish have mysteriously died in the Murray River over the course of the last week, and said the event was likely the cause of 'poor water quality'.
The number is a significant increase from the Department's initial estimate of 3000, and Recfishwest said it was likely the number would continue to grow.
The 'fish death event' triggered a warning from the department last Wednesday, and local anglers were advised to refrain from eating or handling any fish caught in the catchment while water and fish samples were collected and tested.
An investigation into the event was launched, and fish and water samples were collected for further testing.
Department of Water principal scientist doctor Tim Storer said the mass death was most likely caused by poor water quality due to low dissolved oxygen levels.
"Recent rains washed organic matter into the river and disturbed sediment that contains very little oxygen. These two factors resulted in dissolved oxygen falling below levels fish could tolerate," he said.
The Department of Water said it would continue its observations, and conduct fortnightly water quality sampling in the catchment.
A Recfishwest spokesperson said dead fish had been reported around the South Yunderup area, and downstream of Forrest Highway towards the river mouth, and called for 'open, honest and timely' information from relevant government departments.
"When there's a fish kill of any magnitude, and there's multi-agency involvement and plenty of [money] and resources spent, it's not unreasonable for the community to expect to be kept up to date about what is happening and whether it's safe to eat the fish or play in the water - especially with a long weekend upon us."
It is understood dead fish species include bream, whiting, flounder, crabs and mulloway.
The Department of Health continues to advise anglers to avoid eating or handling fish found dead in the river.
Recfishwest have advised local anglers to report dead fish to FISHWATCH on 1800 815 507