Coal firms won the right to claim the planting of grass or trees on old mine sites as conservation offsets for future woodland destruction despite strong opposition from environment department staff, new documents reveal.

The reports detail the 2013-14 internal debate between the Department of Trade & Industry and the Office of the Environment and Heritage (OEH) over a plan that broadened the scope of what miners could count as compensation for habitats wiped out by new mines.

OEH argued in one note, secured by the Nature Conservation Council (NCC) under freedom of information laws, that "there is no certainty that functioning ecosystems can be restored to their original value through rehabilitation" after a mine closed.

"[M]any animal species require resources that are found only in mature forest," it said.