Lee Shearer, the state's resources regulator, told estimates on Friday that the government had begun investigations into "a number of mining title holders, KEPCO being one".

A Planning department spokeswoman confirmed on Monday the investigation involved "allegations of corruption and bribery by its parent company, KEPCO Electric Power Corp in South Korea, and other subsidiaries".

Adam Searle, Labor's mining spokesman who had pressed Ms Shearer on the probes, said the Korean utility had been the focus of well-publicised scandals, including the fabrication of safety tests for nuclear equipment.

In Australia, the company's subsidiary had already been prosecuted for using false photographs to support its Bylong Valley mine application, and the government should have moved sooner to examine the company, Mr Searle said.

"Even in NSW, their track record is not unblemished," Mr Searle told Fairfax Media. "How can the community have confidence that those [fit and proper] standards are being applied?"

Fairfax sought comment from KEPCO.

The planned 25-year project for Bylong, north-east of Mudgee, received a setback in July when the independent Planning Commission found KEPCO's proposal provided "insufficient evidence" that the mine needed to be open cut, among other gaps.

"KEPCO admitted to the offence and entered into an enforceable undertaking, but the company has shown it has no respect for the rule of law, and is willing to cut corners to get what it wants, Warwick Pearse, Bylong Valley Protection Alliance spokesman, said. "KEPCO can't be trusted with a mining licence, and the government should refuse to give them one, and cancel their exploration licence."

Steve Phillips, Lock the Gate's Hunter region coordinator, said local opponents of the mine had sought to raise the issue of KEPCO's standing in a letter set to the government earlier this year.

The corruption allegations abroad and its misuse of photographs in its application in Australia raised "serious questions whether this is the kind of company that should hold a mining licence in NSW", Mr Phillips said.

Developing a coal mine in a prime agriculture region was "a foolish idea for a range of reasons", with water issues among the proposal's "big showstopper" issues, he said.

The Planning department spokeswoman declined to list names of other titleholders being investigated other than to say Ms Shearer "is not conducting any fit and proper inquiries into Shenhua".

Chinese state-owned Shenhua is seeking to develop an open-cut coal mine at Watermark on the Liverpool Plains.