A one-in-100 year rain event would have to occur before a one-in-1000 year earthquake for Myuna Bay Sport and Recreation Centre to be impacted by Eraring power station's ash dam, an Origin Energy representative allegedly said in a briefing to a council committee.
Lake Macquarie councillors Brian Adamthwaite and Barney Langford say they were told at a coastal management committee meeting on June 6 that both events had to occur in order for the centre to be threatened, not just a 5.7 magnitude earthquake as Origin had previously publicly stated.
Cr Langford said the rain event remarks left him "picking my jaw up off the ground".
"I was flabbergasted," he said.
"This was the first time we'd heard this combination.
"I asked for a clarification and the Origin Energy person definitely stated, and repeated, that it was a combination of the two events."
Cr Adamthwaite said he was "surprised" by the previously unmentioned scenario.
"Whether or not the information provided to us was sanctioned by the organisation, I don't know, but it was definitely stated that the two events were necessary for the catastrophic collapse of the [ash dam] wall," he said.
Cr Langford said two prior "high-level" briefings with Origin Energy and the NSW Department of Education had "identified the earthquake as the sole cause of the problem".
Cr Adamthwaite said "a number of people" at the June 6 meeting commented that the rain event reference had never been heard before.
He said the briefing had "raised more questions" about the "process" and "outcome" of the centre's closure.
"I think there are questions to be answered, I don't think we have the full story," Cr Langford added.
"If it requires a combination of two events to occur, why weren't we [initially] told that?
"Why was it only the earthquake that was put forward as the trigger for the closure?"
Both councillors thought the risk to the centre could be managed if both events needed to occur.
"If there were two events, then the warning system could revolve around the fact that after the first event places could be evacuated," Cr Adamthwaite said.
An Origin spokesman did not comment on what occurred at the June 6 meeting.
"A number of scenarios were modelled during our ash dam stability assessment, with a significant earthquake event found to have the greatest impact," he said.
"Contrary to recent suggestions, the assessment did not conclude a seismic event needed to occur at the same time as a significant rain event for the dam to break."
"These assessments are currently being examined by the Dam Safety Committee on behalf of NSW government as part of their independent review into the closure of the Myuna Bay Sport and Recreation Centre."