The research, done exclusively for the Newcastle Herald's Power and the Passion series, was based on an assessment of images from NASA's Landsat 8 satellite and Geoscience Australia from May 2020.

Cartographer Olivier Rey Lescure said of the 2000 square kilometres comprising the Hunter Valley floor, 64 per cent, or 1280 square kilometers, was taken up by mining leases.

About 14 per cent, or 275 square kilometres, of the Hunter Valley floor is, or was, active open-cut coal-mining, compared to 189 square kilometres, or 9.5 per cent, 12 years before.

Mr Rey Lescure said mining rehabilitation looked "green and healthy" after rain, narrowing the extent of active mining from 315 square kilometres, or 16 per cent, measured in 2011.

The area affected by open-cuts increased from 68.9 square kilometres in 1987 and 126.3 square kilometres in 1996. This includes 23 final voids that will be left when open-cut mining ends.

The Upper Hunter final voids include two at Mount Thorley Warkworth mine, which cover nearly 1000 hectares and reach up to 300 metres in depth. The next biggest voids are at Mount Arthur mine near Muswellbrook which will cover 700ha.

An Australia Institute report revealed last year that up to $25 billion could be needed to fill and rehabilitate the Upper Hunter's final voids. By contrast, the state government holds just $3.3 billion in environmental rehabilitation bonds for all mines in the state.

"The mines in the Upper Hunter have approval to leave behind 23 huge holes, known as final voids. These voids affect groundwater and often fill with highly saline or otherwise unusable water," the report says.

The NSW Audit Office warned in 2017 that security deposits held for the state's 450 mines "do not include sufficient contingency given the substantial risks and uncertainties associated with mine rehabilitation and closure".

Sydney University's Professor Phil McManus said it was not sustainable for mines lasting 20 years to leave a legacy of land unsuitable for other use.

The urban and environmental geography professor, who has studied the Hunter coal industry, said the region offered a great lifestyle and had huge potential to attract other industries. "Coal has been very important, you can see why it has been crucial to the region," he said. "But the future is going to be very different from the past."

Without a planned transition to move with the climate-change led global shift away from thermal coal, the Upper Hunter could be left "high and dry".

He said large swathes of the landscape had been "completely changed" by open-cut mining, but it was time to "take mining off the table as the dominant industry" into the future.

"If there was an orderly transition it may stimulate investment in new forms of agricultural infrastructure to generate employment," he said. "Coal is going to be around for some time, but looking ahead to a future the same as we have now means we will inhibit the development of other industries. Removing it as the dominant player into the future could help other industries."

Hunter councils have been calling for years for federal and state government support to unlock mining land across the region for residential and commercial development.

Singleton Council told a federal inquiry in 2019 its ability to transition from coal and a "single engine economy" relies in part on loosening mining industry control over large parts of the shire to stimulate non-mining jobs.

"Mining buffer lands and remediated mining lands present a large opportunity for agribusiness expansion, however miners are not willing to risk mining approvals to allow development on these sites," the council said in its submission to a federal parliamentary inquiry on regional jobs.

The council asked for government support to influence the post-mining use of land so that mining companies plan for future "highest and best use" of rehabilitated and buffer land, in consultation with council, the NSW government and the community.

Lake Macquarie Council has also identified several former mining areas on the lake's west to reduce the cost of delivering new housing.

The council called for federal and state government support to "explore opportunities for the adaptive re-use of ex-mining land", to build on the "positive legacy of coal mining in our city" in the past.

"Using these sites would greatly reduce the cost of delivering new housing, including costs associated with supporting infrastructure, when compared to new greenfield developments on the fringes of the city," a council spokeswoman said. "We have a vision for ex-mining land to be repurposed to support the same, if not greater, economic output achieved from the mine site during the peak of its operations."

The council cited the BlackRock Motor Park approved for the former Rio Tinto Rhondda Colliery at Wakefield as "a demonstration of the potential economic outputs ex-mining land can deliver". The planned project "has the economic potential of $357 million with 461 jobs during construction and $129 million and 229 jobs annually", the council told the federal inquiry.

Under a "high growth scenario", where Lake Macquarie's population jumps from 206,000 in 2016 to 379,000 by 2050, the city will need 76,000 new home lots over 5000 hectares (50 square kilometres), and nearly 900 hectares of land for future employment needs, the council said.

But council identified only 16,760 potentially available lots over the next two decades because of land availability, infrastructure provision and environmental constraints, leading to only a "bare equivalence" in the supply and demand for housing lots, council chief executive Morven Cameron said.

According to Lock the Gate, current mining titles cover about 90,650 hectares in the Upper Hunter, Muswellbrook, and Singleton local government areas. A spokeswoman said not all of this would be under active mining at all times.

"You could call all of this affected by mining, whether its pits, infrastructure, overburden piles, rehabilitation areas, transport, admin buildings, etcetera," she said.

Lock the Gate estimates, based on land titles data, that mining companies own at least 125,580 hectares of land in the three local government areas, of which 63,616 hectares is outside the mining titles, so the total area of land they occupy is 154,267 hectares.

Upper Hunter Mining Dialogue's latest data shows 23,259 hectares of land was disturbed by mining at December 2020. Funded by the NSW Minerals Council with a mandate to address community concerns on the impact of mining in Upper Hunter communities, the group reported 848 hectares was rehabilitated and 1380 hectares was "newly disturbed" during 2020.

The total amount of land rehabilitated in the Upper Hunter was listed was 14,242 hectares, 43,683 hectares was held as biodiversity offsets, 59,160 hectares was managed for agriculture and 48,175 hectares held as buffer zones.

Another contentious part of the mining industry's legacy is mine site rehabilitation. Pressure is mounting on the state government to force mining companies to increase the rate of rehabilitation occurring on Hunter Valley mine sites.

It follows Lock the Gate analysis last year of the annual reviews of four mines - Bengalla, Mt Pleasant, Muswellbrook and Mt Arthur - that found the overall area of active disturbance was 2.3 times the area of land under active rehabilitation.

The average area of active rehabilitation as a percentage of the total footprint was 27.5 per cent. The most recent independent environmental audits for the four mines describe 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.

A NSW Minerals Council spokesman said at the time that all mines were required to meet strict rehabilitation obligations as set out in their conditions of approval and work to ensure disturbed land was progressively rehabilitated.

Mining operators will be required to show their plans for progressive rehabilitation and to report annually on rehabilitation outcomes as part of a series of reforms designed to reduce the environmental impacts of mining.

The amendments to the NSW Mining Regulation Act aim to modernise the framework and to ensure that progressive rehabilitation is carried out over the lifespan of mines.