The mining union has called on the NSW government to secure up to 950 jobs at two Lake Macquarie collieries as it negotiates to keep Eraring power station open beyond next year.
Myuna and Mandalong colliery owner Centennial has a contract to supply coal to Origin Energy's Eraring plant, but the agreement expires in June.
The power station is due to close next year, but the government is negotiating with Origin to keep the plant open, possibly for another four years, to shore up electricity supplies.
The Mining and Energy Union says the Minns government must ensure Myuna and Mandalong continue to supply coal to Eraring as part of any deal to subsidise the plant beyond 2025.
"At least 600 and up to 1000 direct jobs are at risk across the two mines unless a coal supply contract for Eraring is urgently secured," the union said in a media statement.
"Centennial's Myuna mine at Lake Macquarie is a fully captured coalmine of Eraring power station, supplying coal via a conveyor belt with no other opportunity to transport coal to other markets.
"If no new ongoing contract is secured, there is an imminent risk of the mine closing and 350 workers being stood down."
The nearby Mandalong mine contributes about half of its coal to Eraring.
The union said about half of Mandalong's 600-strong workforce could lose their jobs if Centennial could not secure another Eraring contract.
The MEU said Origin had taken advantage of the government's temporary $125-a-tonne price cap on coal by stockpiling supplies from other sources.
The price cap expires on June 30. Newcastle thermal coal is trading at about $US145 ($220) a tonne this month.
"Despite skyrocketing profits over the last 12 months, Origin has rejected attempts by Centennial to negotiate a new contract on viable terms, partly due to stockpiling coal purchased elsewhere under price cap rules," the union said.
The union has launched an online petition calling on state and federal ministers and Hunter MPs to intervene.
MEU northern district president Robin Williams said Origin should "do the right thing" and buy coal from "these local coal mines that exist to supply the power station".
"Further, the NSW government should insist on contracts for the life of the power station if they provide support to extend its operation," he said.
"Both the NSW and federal government have reassured energy communities that they are committed to an orderly energy transition, with affected workers being supported.
"The sudden closure of Myuna and Mandalong mines, with the consequent loss of hundreds of good jobs in our community, would undermine confidence in these assurances."
