AGL has delayed the closure of parts of its coal-fired Liddell power plant in New South Wales to meet energy demand over the summer months.

The company has been under pressure from the federal government either to delay the closure or sell the plant to another operator.

In a statement on Friday, the company said it had informed the Australian Energy Market Operator of the schedule for the closure of the Liddell plant in the upper Hunter region of NSW, and the Torrens A power station near Adelaide.

AGL had previously indicated it would close Liddell in 2022 after 50 years of operation.

“AGL has today informed AEMO that the first unit at Liddell will close in April 2022,” the company said.

“However, following an independent engineering assessment, AGL has determined that the remaining three units will close in April 2023, supporting system reliability throughout the 2022-23 summer months.”

The company had planned to mothball two of the four units at Torrens A by November.

But it said it was seeking the permission of the South Australian government to continue to operate them over the coming summer.

“Under the schedule provided to AEMO, the first two Torrens A units will be closed in September 2020. A third unit will be closed in September 2021 and the final unit in September 2022,” its statement said.

On Friday, the energy and emissions reduction minister, Angus Taylor, said the government’s position on Liddell was “well-known and has not changed”.

“We remain in constructive dialogue with AGL about this, and this is a good short-term announcement,” he said.

“But there is more to be done. We want to see existing coal and gas generation stay in the market, running at full tilt.”

Under the new retailer reliability obligation, which took effect on 1 July, energy retailers are required to meet the demand for power 24 hours a day.

AGL said on Friday it was continuing to work on projects to replace the power that would be lost through the Liddell closure.

“AGL is continuing to progress a series of power firming projects in NSW including the 100MW upgrade to the Bayswater power station, assessing the feasibility of 250MW of pumped hydro at Bells Mountain and seeking approvals for a 250MW gas power station at Newcastle,” the company said.

The Nature Conservation Council said in a statement the Liddell decision was the result of federal and state mismanagement of energy policy.

“We have known about the upcoming Liddell closure since 2015,” its chief executive, Kate Smolski, said. “It hasn’t exactly been under the radar. Liddell is NSW’s most unreliable and most polluting power station. Relying on it to limp though summer is risky and irresponsible. It was outages at two Liddell units that contributed to NSW running out of electricity in February 2017.

“The Morrison government’s incompetence in formulating a coherent climate and energy policy has stymied the investment in wind, solar and storage that should have filled the gap in supply that this decision seeks to fill.

“This highlights the urgent need for NSW to go it alone and develop its own climate and energy policy.”

Earlier this week the NSW energy minister, Matt Kean, told the clean energy summit the state still supported the national energy guarantee, abandoned by the federal Coalition, and would contemplate unspecified unilateral action in the absence of a national policy.

“If the Commonwealth won’t get on board, NSW will consider going it alone,” Kean said.