Plans to develop an open-cut coal mine in the bucolic Bylong Valley lack key information about the impacts on the landscape, water resources and the community, the state's Planning Assessment Commission has found.

The commission's review report into a proposal by South Korean-owned KEPCO utility to develop a 25-year coal mine near Mudgee, found "uncertainty and incomplete information" about the project's risks.

The concerns included "insufficient evidence" that the $1.5 billion mine needed to be an open-cut operation, while the proponent's cost-benefit arguments "lack the rigour and transparency" needed to evaluate them, the PAC said.

Air quality and noise impacts, investigation of Indigenous cultural heritage and traffic impacts from the mine all require further clarification, the commission said.