Mining at the site between Muswellbrook and Singleton will end in early 2023 if Glencore's twin proposals for an eight-month extension into the Barrett pit from 2022, and a much bigger expansion to recover 135 million tonnes from the Barrett pit until 2044, are not approved.

The major expansion proposal, which would see annual production rise from 4.5 million tonnes to 10 million tonnes and disturb 750 hectares of land, is on public exhibition until January 31.

Glencore described the expansion as a "brownfield continuation of the existing Glendell pit" that fits within its commitment in February, 2019 to cap global coal production at 150 million tonnes per year of saleable product.

"The project will occur at a time when production at Glencore's adjacent Liddell Coal Operations, and the Ravensworth East and Glendell mines, have ceased," Glencore said in its application to the Department of Planning.

"The coal produced by the project is 'replacement production' that will help to maintain Glencore's long term production profile."

The major expansion would generate $296.1 million in royalties the NSW Government over the life of the mine, keep 300 workers in jobs and provide a net benefit to the Upper Hunter of $446.7 million, Glencore said.

But the Glendell proposal, and Glencore plans to mine an extra 52 million tonnes of coal from its Mangoola mine at Wybong, where 400 workers face a 2025 deadline if the expansion is not approved, will test the NSW Government on climate change as community calls for action increase in the wake of devastating bushfires.

The Glencore proposals will also renew calls from within the Hunter for federal and state governments to commit to a transition plan from coal. They could also trigger another damaging divide in the region over the loss of hundreds of jobs if the Glendell and Mangoola plans are not approved.

In its environmental impact statement Glencore noted the broader Singleton community "appeared generally accepting of the project due to the predicted positive economic benefits" but "there is a trend in increasing concern around cumulative impacts of mining in the Hunter region, primarily in regard to air quality, intergenerational equity and the prolonged nature of impacts".

The EIS acknowledged the long history of mining in the area had "significantly modified the groundwater environment in the vicinity of the project area and the effects of historical and approved mining operations will further affect these systems over the life of the project".

About 80 per cent of the 135 million tonnes of coal in the Glendell proposal will be exported and produce more than 220 million tonnes of carbon when it is burnt as fuel overseas, but it is not counted towards Australia's commitment under the Paris Agreement. The exported emissions are also not counted under the NSW "aspirational" goal of net zero emissions by 2050.

But environment groups say the Glencore proposals should not go ahead because the Paris Agreement means there is a global "carbon budget" - a limit to the amount of emissions that can still be burnt if the world is to avoid catastrophic climate change - which rules out further mine expansions.

The carbon budget argument was accepted by Land and Environment Chief Justice Brian Preston in February, 2019 when he refused the Rocky Hill coking coal mine at Gloucester.

The Glencore assessments have the potential to be even more challenging for the NSW Government than the controversial Bylong coal mine assessment, when the Independent Planning Commission refused the mine in September for reasons including the contribution to climate change.

Economic benefits accrued to the current generation and long-term environmental costs of approving the mine "will be borne by future generations", the IPC found.

The Bylong decision sparked a campaign by the NSW Minerals Council against the IPC, prompting Planning Minister Rob Stokes to announce a snap review of the IPC and proposed new legislation to stop consent conditions relating to greenhouse gas emissions from export coal being imposed on NSW mining projects.

The proposed legislation prompted a former Land and Environment judge, Paul Stein, QC, to say that all NSW residents "should be alarmed" because "this bill is a dangerous and retrograde step which flies in the face of combating global warming".

In its environmental impact statement Glencore said the Glendell proposal, in isolation, was "unlikely to materially influence global emission trajectories".