Coal miners and regulators in NSW urgently need to adopt the US practice of backfilling open-cut mine pits once mining has ended in order to minimise environmental damage, according to a new report.
While the scale and number of open-cut coal mines has sharply increased in the last 15 years, the report by consultancy firm Energy & Resource Insights (ERI) says there has been no planning for rehabilitating the landscape of NSW's mining regions once the coal is exhausted or mining becomes uneconomic.
The ERI report estimates at least 45 voids covering 6050 hectares have been either planned or approved across NSW, with their total area bigger than Sydney Harbour.
Backfilling these vast open pits will lessen many of the social and environmental risks, and could allow the land to be returned to its pre-mine use.
"In the US, filling in coal mine final voids has been required by law since the 1970s. Yet, in Australia, this is still not the case," ERI said in its report, titled "The Hole Truth".
That failure will result in these empty pits putting pressure on the local groundwater, as they take decades or centuries to fill with water. The water quality in the resultant lakes is typically poor and will become increasingly saline over time.
Around 68 coal mines have been active in NSW between 2011 to 2015, with half of them being open cut mines.
The report, commissioned by the Hunter Communities Network which represents mining-affected communities, said mining companies typically avoid complete rehabilitation at the site citing prohibitive costs and criticises regulators.
"Australian mining companies recognise the benefits of filling in voids. However, in most cases mining companies do not undertake backfilling. Essentially, this is due to cost and the lack of a legal requirement to do so," it said.
Miner-estimated filling up costs ranged from $133 million for the Yancoal-owned Moolarben coal mine in the Western Coalfields to $2.1 billion for the Rio Tinto-owned Warkworth mine in the Hunter Valley.
The report pointed to the example of Whitehaven Coal's recently-constructed Maules Creek mine, where the state government allowed the company to avoid backfilling by citing very high capital costs and future mining potential.
Whitehaven had estimated backfilling costs at up to $813 million.
The report called for a comprehensive approach to rehabilitation at the time of the mine planning decision and including pre-conditions, as in the US, for all pits to be filled, in order to ensure costs are lower.
The NSW Department of Planning and Environment said while current rules require all mining operations to meet strict contemporary environmental standards, it is working on providing greater certainty on mine rehabilitation.
A new policy framework is being developed to enhance the government's management of post-mining land-use, landform and rehabilitation outcomes, a spokesperson said.