The state government last month postponed the release of an independent report into the energy transition amid speculation it would urge extending the life of the Central Coast power station, due to shut in 2025.
But Liberal MP and former state treasurer Matt Kean said advice he had received while in government, from an unnamed energy expert, was that keeping Eraring open for another three years could cost $3 billion in taxpayer-funded subsidies.
This figure was costed when the former coalition government was pitched a deal to buy the power station from Origin.
Premier Chris Minns dismissed the sum and questioned the opposition MP's contribution to the energy transition debate.
"I don't know where he got those numbers from," he said.
"Matt Kean's intervention is completely cynical.
"He's responsible for the fact that renewable energy production is delayed in NSW, it's over budget and he didn't tell anyone before the last election.
"We've been left with a bonfire, effectively, when it comes to energy costs in NSW."
But Mr Kean told AAP it was the premier who was not across the detail.
"What I've done is be up front with the deal that was put to the previous government," he said.
"In an environment where Labor haven't costed the impact and the (energy market operator) says it's not needed, why risk potentially billions of taxpayer dollars when you don't know what the cost is?"
Energy Minister Penny Sharpe said the government "had not looked at" the cost of potentially extending the life of Eraring.
She said the independent report, which was originally due to be released by the end of August, would soon be made public along with the government's response.
"The selling off of our electricity system has made every aspect of this transition harder," she said.
She added the figure Mr Kean quoted was "unverified" and "completely unhelpful".
The state government has not committed to an extension at Eraring, but Mr Minns last month said the plant was critical to the state's energy supply amid power price pressures.
The power station has a generating capacity of 2880 megawatts, about 25 per cent of NSW's electricity needs.
Electricity market operator AEMO in August warned NSW could face energy reliability shortfalls from 2025 after the retirement of Eraring, noting that risk had increased due to higher demand forecasts among other factors.
Environmentalists oppose keeping open the plant, which is Australia's largest greenhouse gas emitter, and say such a move would be a retrograde step in the shift to renewable power.
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