Almost 600 hectares of bushland above the Illawarra Escarpment has been bought by mining company Illawarra Coal.
The large tract of land has been on the market since 2011, after former owner and fellow coal company Sada Services was unable to win permission to develop the land and decided to sell up.
But the new owner may not seek to develop the land.
Illawarra Coal Holdings, a subsidiary of BHP Billiton, bought the 590 hectares for $2.4 million, and says its reasons include biodiversity conservation.
The purchase drew speculation Illawarra Coal may use the land as a "biobank" offset - dedicating the land for conservation, and in return seeking permission to develop environmentally sensitive land elsewhere.
A company spokeswoman told the Mercury any such plans had not been decided.
"Illawarra Coal has purchased land on the Maddens Plains for uses including biodiversity preservation," she said.
"Details regarding its potential use as an offset are yet to be determined," she said.
To the west of the package of land is the Dharawal National Park, and beyond that lie Illawarra Coal's Appin and West Cliff mines.
Substantial areas of the Dharawal were in Illawarra Coal's coal lease, and were originally included in the miner's plans for expanding operations at the Appin mine. But after a scathing assessment from the Planning Assessment Commission, the company excluded these areas from its mining plans.
To the north-west of the package of land is Heathcote National Park, and to the south is the Illawarra Escarpment State Conservation Area.
Environmentalists saw it as the "missing piece" in a large conservation area, home to large numbers of upland swamps and native species, and had campaigned for government funding to help Wollongong City Council buy the land.
Illawarra Escarpment Network convener Carol Berry said the land was of "critical local environmental significance".
"The campaign to bring the lands into public ownership, and therefore ensuring their ongoing protection, attracted overwhelming public support," she said.
"That campaign was unsuccessful, but it is important that the new owners of the land are mindful of the notable public concerns regarding the conservation value, and importance of protecting these lands."
The Mercury has been told the new owner is rehabilitating parts of the land, fixing up its fences.
Sydney-based Sada had previously operated a coal washery on the land. The site of the former washery retains extensive volumes of coal waste.
There is also a dam, which was owned by the Illawarra Coke Company and used for its now-closed coke-making operations at Coalcliff.
Sada had previously sought to develop six houses on the land, but this was rejected by Wollongong City Council as being inconsistent with the zoning objectives.
Sada appealed to the Land and Environment Court in 2009, but that court confirmed the council's decision, saying the development would not enhance environmental protection, and the bushfire risk was too high on some lots.
Within two years of this decision the land was on the market.