The Australia Institute estimates up to $25 billion could be needed to fill and rehabilitate the Upper Hunter's 23 final voids when open-cut mining ends. By contrast, the state government holds just $3.3 billion in environmental rehabilitation bonds for all mines in the state.

A particular challenge in the Hunter is the average depth of the region's mines makes them more difficult to rehabilitate than elsewhere.

"Deeper pits means a progressive rehab is harder; you need to leave your void so you can access the coal at all times," mine closure and rehabilitation expert Ms Karin Fogarty said. "Unfortunately, you've got a very limited footprint of your mine, so the only option you have for your dirt is to go up.

"That's why you end up with these exposed hills, like you see over in the Hunter, where, by comparison, somewhere like Mudgee has a much shallower resource, you can effectively mine a strip and then you can dump your overburden directly into the strip behind it."

While meeting compliance goals may still underpin end of mine planning and rehabilitation, broader social and economic factors now play much more significant role in the rehabilitation process.

"There's been this huge paradigm shift and that might be driven out of the fact that there's this realisation that the mines are going to close sooner rather than later and certainly in areas like the Hunter where we have such a high number of mines," said Ms Fogarty, who has previously worked as an environmental advisor with Rio Tinto and Peabody Energy.

"It's really going to make an impact on the surrounding communities. Broader stakeholder demands have certainly intensified as well; communities are really looking for a greater societal contribution from the mining companies. Just meeting compliance obligations is just no longer enough.

"On the other side of the coin, shareholder demands on mining companies have also increased. A number of mining companies are using their rehabilitation as a way to demonstrate their contribution to society at large."

Ms Fogarty is completing a PhD in environmental economics at the University of Western Australia.

Her research highlighted a desire from communities to play a greater role in mine site rehabilitation.

In a recent example, Bylong Valley residents' criticism of the environmental impacts of the proposed Kepco open cut mine played a key role in the Independent Planning Commission's rejection of the project.

In their submission, residents said the loss of 400 ha of prime agricultural land consisting of fertile soils overlaying good groundwater could not be rebuilt on rehabilitated land.

"There is no precedent for this scale of prime land reconstruction anywhere. Likewise the proposed reconstruction of 65 hectares of critically endangered ecological community on the rehabilitated open cut mine site is an unproven risk," the residents' submission said.

Governments and industry have been grappling with the vexed issue of mine site rehabilitation since the acceleration of open cut mining in the 1960s. New requirements imposed by the NSW Resource Regulator require mining companies to regularly report on rehabilitation progress and provide updated rehabilitation management plans.

A 2021 Lock the Gate analysis of the annual reviews of four mines - Bengalla, Mt Pleasant, Muswellbrook and Mt Arthur - highlighted this goal remains a work in progress.

The analysis showed the average area of active rehabilitation as a percentage of the total footprint was 27.5 per cent.

Independent environmental audits for the mines described rehabilitation efforts compromised by inadequate human resources and systems.

Poor results from drone seeding, erosion and weed infestation were cited as evidence of a lack of personnel dedicated to rehabilitation management.

Lock the Gate spokeswoman Georgina Woods said taxpayers would be left with a hefty clean-up bill unless the NSW government enforced rehabilitation requirements.

"This data exposes the NSW government's failure to ensure mining companies are cleaning up the mess they are creating," she said.

"It is especially concerning in the context of mounting uncertainty about future demand for coal and the risk of sudden mine closure without plans to keep people employed in land restoration. The pace of rehabilitation is woefully inadequate, and we fear the NSW taxpayer will be left to pick up the bill when these companies either go bust, or decide coal is no longer profitable and mothball their mines."

A NSW Upper House inquiry into the sustainability of the state's energy supply and resources found that mine site rehabilitation could play an important role in the region's energy transition process.

"The work uses many of the skills that the local workforce already has, and the work location is at or near existing job sites. Because of this, we heard that mine rehabilitation could be a useful short-term jobs 'bridge' for semi-skilled workers employed in coal mining and power generation who might struggle to find alternative employment," the report said.

"Mandating rehabilitation will help to ensure proper planning and funding, which will unlock the employment potential of these projects. It's vital that these projects are backed by significant funding from both mine operators and government. The biggest limit on the effectiveness of rehabilitation is funding, which is shown to limit how many jobs they create."

Ms Fogarty's research into post-mining land use trends in NSW and Queensland found that agriculture and biodiversity were the most commonly proposed post-mining land uses.

Agriculture, which once accounted for a large proportion of the Hunter's economic activity, was the most dominant pre-mining land use. There are also an increasing number of sites, such as the Maxwell Coal site south of Muswellbrook, being earmarked for clean energy projects.

Ms Fogarty said consideration about future land use needs in an area like the Hunter was also starting to become more prominent in discussions about post-mine planning.

"I think it's also important to note that mine rehabilitation and the associated post mining land use is, again, just a land use in time," she said.

"There's no real reason that once mine rehabilitation is completed that it can't be then developed into other land uses. But it does require that bit of a forward planning approach to understand what the region's societal needs will be.

"I think that's a particularly pertinent issue in communities like Singleton and Muswellbrook which are expected to grow considerably over the next 20 to 25 years and that will involve moving closer to some of these surrounding mine sites. I think there's going to be a real focus on the need for good planning to make sure we don't waste our efforts creating biodiversity which is ultimately going to be used for urban development."

The state government announced last year that it would invest $107.7 million into the remediation of the state's abandoned mine sites as part of an expansion of the Legacy Mines program.

Legacy mines typically operated prior to the introduction of requirements for comprehensive environmental management and rehabilitation plans. No individual or organisation can be held responsible for their management.

A network of abandoned underground mines across the Cessnock coalfields that still leach raw acid into the surrounding environment decades after their closure are among the worst examples in the state.

Former deputy premier John Barilaro said remedying legacy mine these sites would help accelerate economic growth, support jobs and ensure mining communities remain great places to live and work.

"Regional communities were built on the back of mining projects, but these legacy mines were operating at a different time, well before the procedures and requirements we have today for managing chemical use and rehabilitation were put in place," Mr Barilaro said.

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