Thousands of dead fish have been washing ashore on Mallacoota beaches and parts of the NSW and Victoria east coast since the weekend.
Leatherjackets measuring up to 10cm were sprawled across Quarry Beach, while small numbers of whiting have also been reported.
Environment Protection Authority Victoria said the fish kill was likely caused by recent changes in current patterns and drops in water temperatures measuring 6 to 7 degrees over the past week.
EPA Gippsland manager Stephen Lansdell said Fisheries Victoria and Parks Victoria were in Mallacoota on Wednesday to confirm fish kill estimates.
“It’s definitely not a trawler dump due to the spread, location and the way they’re appearing on the beach,” Mr Lansdell said.
“From an EPA perspective the fact it’s not regarding pollution lowers our main concern, but any fish kill event is still concerning.”
Mr Lansdell said he was first notified of the event about 5pm on Tuesday from a local authority.
“It would have helped us to understand what was going on if we found out earlier,” he said.
“We rely on eyes and ears from the community.”
A similar fish kill event happened on Queensland’s Sunshine Coast last month when thousands of beached leatherjackets were found between Coolum and Cooloola.
Mr Lansdell said it was likely the big drops in water temperature have been tracking south along the Australian east coast.
Mallacoota residents reported some fish were still alive and carrying eggs, while a turtle was also found.
Beached leatherjacket sightings have also been spotted as far north as Tura Beach.
Mallacoota and District Angling Club president Col Gilchrist said it was the largest local report of fish kill he had heard of.
EPA Victoria encourages residents to contact the department of any similar or strange sightings on 1300 372 872.