Forest and Bird has won an appeal against the development of an opencast coal mine on the West Coast.
Stevenson Mining had wanted to get access to reserve land held by the Buller District Council for its new mine which would cover 109 hectares in total, including 12 hectares of conservation land.
Under the operating name of Rangitira Developments, it wants to open a new mine at Te Kuha, about 12km up the Buller River from Westport, but has been fought every step of the way.
The company had successfully argued in the High Court that the Council had to factor in economic benefits of the mine when considering this access.
But Forest and Bird appealed this and won.
The group's lawyer, Peter Anderson, said the council was bound by the Reserves Act and needed to protect the environment.
He said this sets a precedent for future coal mines on reserve land.
"The natural and biological features of this reserve have to be protected, and that's fabulous news for the plants and animals up there because it's a unique ecosystem and so protecting those values is kind of inconsistent at first glance with an opencast coal mine," Mr Anderson said.
It is the latest blow for Stevenson which has been developing its plan for the Te Kuha mine for years.
In June the government turned down its application to mine on the conservation land.
The company has said it intends to apply for a judicial review of this decision.
Stevenson Mining's chief executive, Mark Franklin, said it was too early to say what the latest court decision meant for the mine.
He said it would be discussed by the board early next month.