The extension will allow the mine to run for another eight years and produce 52 million tonnes of coal.

In her decision, Ms Ley said the mine would contribute approximately 0.00073 percent to global emissions per annum.

"Based on this estimate (the mine)... is unlikely to influence global emissions and climate change trajectories," she said.

Glencore welcomed the approval, which will secure employment for the mine's 400 workers and create another 100 jobs during construction.

The company said the coal mined in coming years would offset closures and lower output at its operations elsewhere and had been included in its plans to halve its direct, indirect and customer emissions by 2035.

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But Lock the Gate Alliance slammed the approval and said the federal government was "hell-bent on approving as many coal mines as it can before the Glasgow climate conference, which begins next month."

Spokeswoman Georgina Woods said Ms Ley had abandoned Australian children who would be forced to live with the climate harms she had helped create.

"Glencore's Mangoola coal mine expansion was opposed by Muswellbrook council and local landholders because of its unacceptable social and environmental impacts," she said.

"But the Morrison Government seems more interested in allowing Glencore to make a few extra million out of exploiting the Hunter Valley than it is in the welfare of the community and the land."

"These are the third and fourth coal mining projects (the Mangoola extension and the Tahmoor metallurgical coal mine south west of Sydney) the Australian Government has approved in recent weeks despite a recent Federal Court judgement acknowledging the harm to human health, and particularly to children.

"These decisions come just weeks before pivotal global climate change negotiations at Glasgow. It is frankly alarming to witness this country digging itself deeper into its obstinate refusal to take climate change and rural sustainability seriously.

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