THOUSANDS of hectares of pristine wilderness on the edge of south-western Sydney, fought over by mining and environment interests for decades, will be saved and opened to the public as a national park if the NSW Coalition wins government in March.

But the announcement, to be made today by the Opposition Leader, Barry O'Farrell, sets up a potential conflict with BHP Billiton, which still holds the right to mine the billions of dollars worth of coal in the area.

The park, based on the Dharawal State Conservation Area, would add about 6200 hectares of bushland to the public estate, and add one of the final pieces to the jigsaw of protected wilderness areas around Sydney.

It is one of the last remaining areas of intact natural bushland near the city, and sits between the Royal National Park and the Blue Mountains World Heritage Area.

Mr O'Farrell said ''no deal has been done'' with the mining company, which revised plans to mine under the conservation area in October but retains the right to apply again over the 30-year life of its lease.