The Morrison government has been accused of demonstrating it is not taking the climate crisis seriously after approving a third new coalmine development in a month shortly before a major international conference on the issue.

With global climate talks in Glasgow less than four weeks away, the environment minister, Sussan Ley, has given a subsidiary of the mining giant Glencore the green light to expand the Mangoola mine near Muswellbrook.

It means the company can create a new coal pit north of an existing mine to extract of an additional 52m tonnes of the fossil fuel over eight years.

The mine had been approved by the NSW Independent Planning Commission in April, less than a month before a state byelection in the Upper Hunter in which the future of the coal industry was a prominent issue.

Conditions attached to the federal approval require the company to monitor local water and offset the destruction of habitat used by species including the critically endangered regent honeyeater and the vulnerable grey-headed flying fox.

Margot White, who runs a small beef farm with her husband Michael 2.5km from the planned pit site and is a member of the Wybong Concerned Landholders group, said she was unsurprised but bitterly disappointed by the decision.

“At some point, we need a government that is going to be brave enough to say there are not going to be thermal coalmines any more,” she said. “It’s a great shame … I just think it’s incredibly short-sighted.”

In a published statement of reasons for her decision, Ley said she had considered the findings of a federal court case – known as the Sharma case – which found she had a duty of care to protect young people from the climate crisis. She repeated an explanation she gave for her approval of the Vickery mine extension near Gunnedah and the expansion of underground mining at the Russell Vale Colliery near Wollongong.

Ley wrote that she had accepted advice from her department that the Mangoola development was “not likely to cause harm to human safety because, if the proposed action is not approved, there is no reason to believe that a comparable amount of coal would not be consumed in substitution of the proposed action’s coal”.

“Therefore, I considered that the proposed action will not necessarily result in an increase to global GHG (greenhouse gas) emissions,” she said.

Ley has lodged an appeal in the Sharma case.

Georgina Woods, the Lock the Gate Alliance’s New South Wales coordinator, said the project had been opposed by Muswellbrook council and local landholders due to its unacceptable social and environmental impacts. She said the Morrison government’s decision to approve it showed a lack of regard for the welfare of the local community.

“This is the third coal mining project the Australian government has approved in recent weeks despite a recent federal court judgment acknowledging the harm such projects will do to human health, and particularly to children,” she said.

“Coming as it does just a month before pivotal global climate change negotiations, it is frankly alarming to witness this country digging itself deeper into its obstinate refusal to take climate change and rural sustainability seriously.

“This decision is the result of the Morrison government’s refusal to have any kind of valid climate change policy and it will be Australian children and rural communities that will pay the price.”

Under the project plans, the expansion would have a 623-hectare footprint. White said the new pit would leave an 82 ha void in the landscape.

“It beggars belief, really,” she said. “It seems there is such a large, permanent, negative environmental impact for such a short-term gain for so few.”

Glencore said it had invested more than five years in detailed studies and consultation for the project, which would create more than 100 construction jobs and secure ongoing employment for Mangoola’s workforce of approximately 400 employees.

“The coal mined at Mangoola over the next four years will offset closures and reduced output at a number of other Glencore operations, including Newlands (2023), Liddell (2023) and Clermont (2026),” the company said.

Glencore said continued operations at Mangoola had been factored into its August 2021 announcement that it would introduce a short-term emissions reduction target of 15% by 2026 and increase its medium-term target to 50% by 2035.

A spokesperson for Ley said the minister was carrying out her responsibilities in accordance with national environment law, and taking into account “all other relevant considerations”. They said the approval followed a rigorous assessment process and included strict environmental protection measures.

The approval comes as the Morrison government remains split over whether to set a target of reaching net zero emissions by 2050, or to increase its 2030 emissions reduction target, before the Cop26 climate summit in Glasgow.

More than 120 countries have set the net zero target and leading developed countries, including the US, Japan, Canada, Britain and members of the European Union, have all increased their 2030 goals significantly in the past year. International and domestic political and business leaders have urged the Morrison government to join them.

As debate about climate within the government continues, it has made several announcements of public funding for fossil fuel projects.

On Tuesday the emissions reduction minister, Angus Taylor, confirmed that the government had granted $30m to Australian Industrial Power – owned by the billionaire Andrew “Twiggy” Forrest – for work on a gas-fired power station at Port Kembla.

Last week Taylor announced that fossil fuel projects that promise to capture and store carbon dioxide emissions underground would qualify for carbon credits, and could bid for taxpayers’ support from the $4.5bn emissions reduction fund. He also committed $250m to design carbon capture and storage hubs and support its research and commercialisation.