Hundreds of dead fish consisting of multiple species continued to wash ashore on Monday at Mannering Park and beyond.

Environment Protection Authority investigators collected fish and water samples for testing.

Investigating officers noted greying around the gills of some fish, possibly pointing to oxygen depletion.

While initial investigations point to a lack of oxygen, the root cause remains unclear.

Black water events, such as those that occurred in Newcastle's Throsby Creek earlier this year and in the Hunter River at Easter 2021, occurred due to a combination of warm weather, a sudden influx of fresh water and the presence of large quantities of dead and decaying vegetation.

None of those factors appeared to be a play in the lake on the weekend.

There have also been several fish kills in the Myall Lakes in recent years, which have been caused by the Myall River's inability to flush effectively into Port Stephens.

Delta Electricity, which operates Vales Point Power Station said there had not been any abnormal operations at the plant to suggest it may have contributed to the incident.

Company secretary Steve Gurney said Delta had offered its assistance to the EPA investigation.

Jason Nunn, from Fisherman's Warehouse at Marks Point, said the episode was incredibly disturbing given the good health of the lake in recent years.

"There's a few things here that just don't add up; it's certainly not clear cut," he said.

"We had a bit of rain but nothing like we've had in recent months. The other thing is the water that comes out for the outlet pipes at the power stations is highly oxygenated and turbulent.

"The fact that there are still dead fish coming to the surface today suggests that it wasn't just a quick hit. It's possibly ongoing."

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