The Department of Conservation says a new open-cast mine proposed for the Denniston Plateau north of Westport would cause significant and in some cases irreversible harm to the area.
On Thursday, Conservation Minister Nick Smith gave the Australian mining company Bathurst Resources access to 106 hectares of conservation land on the plateau.
Its use for mining is still dependent on resource consent, which was granted in 2011 but is currently subject to appeal by conservationists.
In advice to the minister published on its website on Friday, DoC says an open-cast mine would cause significant loss of key flora, destroy natural landforms and permanently alter soils.
The advice also says the mine is inconsistent with the objectives of the Conservation Act and the purposes for which the land is held.
Despite its comments, the department makes no recommendation to the minister on whether Bathurst should be allowed access to the land.