The Turnbull government's plan to keep the worn-out Liddell power station running for another five years would cost about $1.4 billion more than replacing it with clean energy, and spew millions of tonnes of damaging carbon pollution, a new analysis shows.

The findings cast further doubt on the wisdom of keeping Australia's oldest operating coal plant open beyond its slated closure in 2022, and have implications for the expected retirement of most existing coal-fired power stations within 15 years.

Prime Minister Malcolm Turnbull and Energy Minister Josh Frydenberg in September ordered energy giant AGL to keep open the coal-fired plant for five extra years or sell it to a party that will.

The demand followed a report by the Australian Energy Market Operator that warned the closure of Liddell, in NSW's Hunter region, would leave a 1000-megawatt shortfall of "flexible, dispatchable" capacity, which is energy that can be created on demand.