On June 26, the EPA was alerted to thousands of dead bream on the riverbank and ordered an investigation into their death.

While the investigation is in its early stages, the EPA believes the mass fish death may have been a natural occurrence.

It is the second mass fish death in two months in the Scamander River, following another incident in May.

But recent dredge work at the river's mouth is likely not the explanation, a spokeswoman said.

"It's unlikely that this work caused the fish kill incident. A previous fish kill was reported from Scamander River in May, prior to the work being undertaken at the barway, and dead fish from the June incident were reported several kilometres upstream from the river mouth," she said.

A Scamander recreational fisher of 20 years, who did not wish to be named, believes there may be more to the deaths than meets the eye, however.

He said many residents have stopped fishing in the area and sea birds have not touched the fish carcasses which remain along a five kilometre stretch of river.

He has also documented the presence of a strange white powder that currently coats river weeds and rocks on the bank.

"If birds are turning their nose up at the fish, then I think this is more than just a natural occurrence.

"I've lived here for 20 years and I've never seen this powder on the banks like this."

He believes there may be a link between recent water pipe maintenance in the area, the white powder and the mass fish death, although TasWater has denied the claims.

"We are confident this is not related to sewer spills or sewerage maintenance," a spokesman said.

"TasWater cleaned drinking water pipes using a technique called ice scouring. There have been no cleaning processes undertaken with sewerage pipes. The waste from the ice scouring was collected and transported to one of our wastewater treatment plants, in this case Georgetown."

The EPA's investigation is continuing.