A plan to extend the life of a major mine to 2050 has been described as the "largest coal-mining proposal ever put forward" in NSW with warnings steep cuts to other sectors' emissions will be needed to compensate.
The proposal to continue Hunter Valley Operations, a joint venture between Glencore and Yancoal, would produce an extra 437.6 million tonnes of thermal coal and 59.7 million tonnes of metallurgical coal for export.
The NSW Environment Protection Authority warns the project has "significant unabated greenhouse gas emissions".
"This proposal is the largest coal mining proposal ever put forward in NSW," a letter penned to the NSW Department of Planning, Housing and Infrastructure reads.
"In its current form, the modelling indicates this proposal is likely to make up approximately 14 per cent of future new coal production in NSW between 2024 and 2050 and has significant unabated (greenhouse gas) emissions."
The environmental watchdog warned the project heavily relied on offsets, but its backers had provided no information about where the offsets would be generated.
"If approved, modelling indicates it will account for approximately 35 per cent of all NSW coal-sector greenhouse gas emissions in 2045 in the absence of any on-site abatement or offsetting in NSW."
That would represent more than 3.5 per cent of forecast total emissions NSW in 2045.
Without changes to emissions or abatement, the increase would mean other parts of the NSW economy would be required to compensate with deeper cuts for the state to meet its legislated targets to reach net zero by 2050.
The EPA recommended mining beyond 2040 should be restricted "due to the significant fugitive emissions that will be generated from the deep coal reserves".
The warnings come as forecasts show NSW is falling short of its interim goals for 2030 and 2025.
Anti-coal mining campaigners Lock the Gate Alliance said approving the largest coal project in the state's history would blow the targets out of the water.
"Other industries and businesses in NSW should not have to bear the brunt of greater emissions reduction because coal mines, the biggest cause of global warming, are still trying to expand," NSW co-ordinator Nic Clyde said on Friday.
But Hunter Valley Operations noted the environmental watchdog was not calling for the proposal to be rejected and questioned its assessment of the mine's share of NSW greenhouse gas emissions.
General manager Dave Foster said the proposal would be to continue mining largely in already approved areas with little additional disturbance and no increase to annual production volumes.
"Continued mining at HVO will provide the community with an economic and social anchor as we move to a lower-carbon future," he told AAP.
The warning comes as a recently released document revealed Origin Energy offered to sell its Eraring power station to the NSW government for $544 million in 2021 before announcing it would close the plant in August 2025.
The nation's largest coal-fired power station will stay open for at least another two years under a deal between the company and the state Labor government, which came to power in March 2023, that could cost taxpayers up to $450 million.
