A five-kilometre stretch of the riverbank was lined with the dead fish in late June, causing concern for residents in the area - many of who use the river for recreation.

A multitude of rumours have since been quashed by the Environmental Protection Authority, who has ruled that the fish most likely suffocated due to low levels of dissolved oxygen. Similar incidents have been reported in rivers across the state in recent years.

The EPA admitted there was no way of knowing exactly what caused the Scamander incident.

"The most plausible theory is that the fish deaths resulted from natural causes. A possible reason is that the fish became trapped in the deeper more saline waters and died from lack of dissolved oxygen. Thermal stress may have contributed to the fish kill as ice sheets on the water surface were observed," EPA director Wes Ford said.

"Results of this testing have failed to provide conclusive evidence of the cause of death of the fish, or of any pollution event linked to the fish deaths."

The three-week investigation - conducted with the help of Break O'Day Council - hit a snag not long after the initial testing of the river's water began when residents reported a white powder on the rocks and weeds on the riverbank.

The substance was found to be predominantly silica-based, naturally occurring and non-toxic.

"The fish pathology report found nothing to indicate the cause of death. No pesticides were detected in the water samples provided, and the levels of metals and nutrients were no higher than expected.

"Stratification of the waters in the lower reaches of the Scamander River is a natural response to seasonal weather, including high rainfall and cold temperatures, and decomposition of naturally sourced materials in the deeper saline waters may result in hypoxic conditions."