The head of the state's environmental watchdog says the authority won't hesitate to take action against Vales Point power station if it is found to be the cause of the latest Lake Macquarie fish kill.
Many in the community believe the incident is connected to the nearby power station.
EPA chief executive Tony Chapel said the authority would "absolutely" act if the Vales Point power station was found to have a case to answer.
"This kind of pollution and environmental damage are prohibited. If there is a particular source, whether it be toxin, a heavy metal or some other contaminant, we will act very quickly," he said.
Delta Electricity has denied any wrongdoing and is assisting authorities investigate the incident.
"I've heard multiple theories from various sources who appear to be jumping to their own conclusions. It's all speculation," company spokesman Steve Gurney said.
"We are still investigating to see if we can identify a cause and the EPA are conducting their own investigation - they are the only facts at the moment."
The EPA investigation into the August fish kill concluded that the incident was most likely caused by natural causes.
Mr Chapel said he shared the community's dismay at the sudden death of hundreds of fish in the southern end of the lake.
"This is devastating for the community; it's depressing to see large quantities of marine life die. We take it extremely seriously," he said.
"We don't have any results yet from our testing and analysis but will share them with the community when we do."
Marks Point Fisherman's Warehouse owner Jason Nunn said he was concerned about the long-term impact of the two recent fish kills on the lake's marine life.
"I would say a fair percentage of the lake's fish have died in these incidents. The reason I say that is at this time of year, and certainly at the time of the first kill, a lot of fish are in the southern end of the lake," he said.
"We can't afford another kill. The size of the fish killed in the latest kill seem to be smaller, so it's a generational killing off of marine life. It's going to make a dent in the fish stocks.
Swansea MP Yasmin Catley called for all available resources to be used to determine the cause of the latest fish kill.
In addition to EPA staff, planning and fisheries staff are also working on the latest investigation. Data loggers have also been put into lake to collect data on temperature changes and salinity.
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