They want the fish stocks replaced and have urged authorities to ensure the cause of the deaths is pinpointed, so it doesn't happen again.

Fisherman Darran Budden believes fish died in the power station's hot water outlet canal before being washed onto the shores of Wyee Bay in southern Lake Macquarie.

"The power station was the cause, I saw the dead fish coming out of the canal," Mr Budden said, adding that video evidence backed up his comments.

"I'm not against the power station, I've worked there. I just want the fish stocks replaced."

Sue and Kelvin Wynn, who have lived in Mannering Park for decades, don't buy the NSW Environment Protection Authority's line that the first fish kill in early August had a "natural" cause.

"You can't tell me the power station is not the culprit," said Ms Wynn, a former Wyong Shire councillor and Greens candidate.

The EPA released a fresh statement on Tuesday, saying that Delta Electricity - owner of the coal-fired power station - had provided the EPA with "new information relating to the recent fish kills".

"The EPA is treating this as a top priority and is using all available resources, including assigning specialist investigators to the issue."

The Herald asked the EPA what this new information was, to which it replied: "To maintain the integrity of the ongoing investigation, the EPA is not able to provide any further comment".

After the second fish kill, video emerged showing dead fish coming out of the canal. Delta Electricity workers were also caught on film around sunrise, allegedly after the fish kill occurred overnight.

As the morning progressed, they were seen scooping fish from the lake and putting them in clear plastic bags and a Delta ute.

These videos have been widely discussed in the Mannering Park community, with questions raised about how Delta staff knew about the fish kill before the sun came up. EPA and Fisheries staff weren't on the scene until the afternoon, according to residents.

One Delta worker was allegedly seen scooping fish from the bay in front of a popular boat ramp and public reserve, about 1.5 kilometres away from the outlet.

Residents were wondering why Delta staff were cleaning up when it was Central Coast Council's job. Delta explained this by saying its staff were helping with the clean up.

"Rather than leave fish to rot on the shoreline, Delta - as a good local citizen - deployed some staff to assist with the clean-up," Delta said in a statement.

Delta said it had continued to investigate the "potential causes of the fish kill" on September 6 [the second fish kill] and "openly co-operate with the EPA as part of its investigations".

"Like the local community, Delta wants to understand what occurred on September 6," Delta Electricity CEO Greg Everett said.

Mr Everett said Delta had continued to inspect and test plant and equipment.

"There is no evidence at this point to conclusively identify a reason for the event. All plant controls and monitoring indicate that plant operated normally and water testing was within normal limits," he said.

"Nevertheless, Delta has taken the step of advising the EPA of potential leads for investigation, so that it can be present during the removal and testing of equipment."

Mr Budden said fishers were seriously concerned about the fish kills because fish spawn in the canal area and then "swim around the whole lake".

"So this affects more than just one part of the lake."

Based on calculations of counting dead fish along the shore, he estimated that up to 12,000 fish died in the first fish kill and about 3000 in the second kill.

He said the first fish kill included a lot of bream and the second was mostly mullet.

"We want these fish stocks replaced and we don't want this to happen again," he said.

Mr Wynn, who fished Lake Macquarie commercially for decades before the practice was banned in 2002, said: "In my years fishing the lake, fish kills were always something that happened up at the power station".

He said if the fish kill wasn't caused by the power station, then "why didn't fish die elsewhere in the lake?"

Ms Wynn agreed, saying "it can't be that the fish kills only happened on the side of the bay with the power station's hot water outlet canal".

She believes the power station canal may have had a fault that led to the fish kill.

Ms Wynn, who is secretary of the Mannering Park Progress group, said "we don't think the EPA fully investigated last time".

After the first fish kill, the Herald asked how many fish were killed but the EPA said "we do not have a precise figure". The EPA maintained that position this week.

After investigating the first fish kill in early August, the EPA said it did "extensive water sampling, toxicity testing and analysis" and there were "no significant findings of pesticides, metals or toxicity".

It concluded that the incident was "likely a natural event".

EPA director regulatory operations, David Gathercole, did concede that "the bay at Mannering Park has elevated temperatures compared to the rest of Lake Macquarie".

This was due to the "cooling water discharges from Vales Point power station". Most people refer to this as the power plant's "hot water outlet".

Mr Gathercole said the first fish kill was "preceded by cool nights in the first week of August and a shift to northerly winds, with the strongest winds on August 4".

"This may have caused cold lake water to enter the bay and resulted in a turnover of the water column. Our results suggest the combination of the temperature and sulfide stress likely overwhelmed fish, acclimatised to higher temperatures."

After the second incident, though, Swansea MP Yasmin Catley said "having two significant fish kill events within a matter of weeks is extremely concerning and distressing for the community". Ms Catley added that it was "unprecedented in Lake Macquarie".

And fishermen who fish around the canal don't think the "cold water and lake turnover" theory makes sense, given the fish were seen dead in the hot water canal.

Residents want to ensure the fresh investigation is thorough.

Ms Wynn said the population in the Mannering Park area had risen, along with property prices. An increasing number of people are aware of the issue and they want the truth. "They aren't going to go with the flow."

Mr Everett said Delta "takes its environmental compliance seriously".

"No one wants to see these events occur. No conclusions should be drawn until every possible lead is properly investigated."