New research from Monash University has for the first time ever quantified the size of the 'final voids' of unfilled mines, which include six mines, current and planned for the Gunnedah minefields including Boggabri, Maules Creek, Tarrawonga, Watermark and Rocglen.

The report states mining companies often object to back filling open cut mines, due to the possibility that additional extraction could be prevented.

Its research states that back filling of these mining voids would only have a "slight impact" on the future of the mine's economics.In the case of Shenhua’s proposed Watermark Mine, the report says the mining giant's plan to leave one of the mine’s three pits unfilled was because if “underground mining is entertained follow-ing the completion of open cut mining it will provide access for this purpose”.

It also stated Shenhua determined “the cost/benefit analysis found that the retention of a safe and stable final void … was the most appropriate outcome”, rather than a filled in void."

Steve Phillips from the Lock the Gate Alliance said with 39 open voids from coal mines approved across the state, the "toxic legacy" of the pits would continue for centuries to come.

“Mining companies are being allowed to leave considerable swathes of NSW polluted and pockmarked," Mr Phillips said.

“Despite the serious damage done by open cut coal mining to farmland, water and bush land, from the Hunter Valley to Western NSW and the Liverpool Plains, the government has never conducted a proper cumulative impact assessment.  It speaks volumes that the community had to commission one instead.

“While experts recommend that all pits be backfilled, regulators have caved into the demands of mining companies and allowed them to take the cheapest and most damaging option of leaving the pits unrehabilitated, which will pose environmental risks for decades, even centuries, after a mine closes." 

The report recommends that the State Government should issue a notice to operating mines that 45 planned final pits should be fully backfilled and a ban on approving any more final voids should be put in place.