Current environmental bonds will be totally inadequate to fully rehabilitate the Hunter's 23 final voids left once open cut mining ceases.
The NSW Government currently holds $3.3 billion in bonds but a report by the Australia Institute estimates that between $11b and $25b would be needed to fill in the voids.
Among the voids are two at Mount Thorley Warkworth mine near Singleton 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.
These voids affect groundwater and often fill with highly saline or otherwise unusable water, according to the report 'Mind the gaps Unused capacity and unfunded rehabilitation in Upper Hunter coal mines' written by Rod Campbell Liam Carter for the Australia Institute.
The NSW Government holds around $3.3 billion in security bonds paid by mining companies to "cover the full cost of all rehabilitation and mine closure activities required if a mining company defaults on their rehabilitation obligations according to the NSW Resources Regulator.
When applying for planning approval, mines typically argue to be able to leave voids because filling them in would cost too much money and occasionally estimates of this cost the report states.
The report listed what was stated by the United Wambo project,' The cost to backfill both voids inclusive of all costs associated with material movement, rehandling, rehabilitation, drainage infrastructure, maintenance and production staff and overheads was $777 million. This equates to $7.0 million per ha for the additional land area gained from backfilling the voids.
Using that information and others from MTW the authors conclude that 'Based on void filling costs of between $2.37 million per hectare, as suggested by the Mt Thorley-Warkworth assessment, and the $7 million per hectare, as stated by the developers of United Wambo, a basic estimate of the cost of filling in the Upper Hunter's 4,457 hectares of voids can be derived."
"The lower end would be $11.5 billion, the upper end $25.3 billion.7 It is clear that the current rehabilitation bonds held are not adequate to even begin addressing the Upper Hunter's voids should current or future generations want to".
The report also investigates the under-capacity of the Hunter's existing coal mines suggesting the Upper Hunter Valley extracted 150 million tonnes in 2020, 91 million tonnes less coal than their approved capacity of 241 million tonnes.
"The fact that the Hunter's coal mines are producing nearly 100 million tonnes per year less than they are approved to shows there is absolutely no need for any new coal mines," said Rod Campbell, Research Director at the Australia Institute.
"This research undermines the case for new coal projects as existing approvals can easily meet existing and likely future demand. The key reason for this gap is that the world is not demanding all this coal.
"The NSW Government's own data shows that Hunter coal sales peaked in 2014. The world has been telling us for years that it intends to use less coal in the future, but NSW Governments have not been listening.
"Instead, NSW governments have been listening to coal companies and their claims that they can't afford to clean up the mess they leave behind."