"This is great news. I was a little bit apprehensive this morning but to have this decision is a massive relief," Bylong Valley Protection Alliance president Phillip Kennedy said.

"It's a big job to fight a High Court battle and there's not many of us left in Bylong. So I'm glad they dismissed the decision before we got to that stage."

KEPCO bought-up around 13,000 hectares of land in the Bylong Valley in central western NSW from 2010, with hopes of building a new coalmine.

As well as the land, they purchased a $400 million exploration license, the Bylong General Store, primary school and Catholic Church - all of which are now shut.

According to KEPCO, the proposed coal mine would be a combination of open-cut and underground operations and would operate for a total of 25 years, extracting 6.5 million tonnes of coal per annum.

"This project would have generated over 200 million tonnes of greenhouse gas emissions," NSW Environmental Defenders Office (EDO) managing lawyer, Rana Koroglu said.

"It would have been an affront to global efforts to limit climate change, particularly when South Korea itself has recently made strong commitments to reduce its greenhouse gas emissions.

"We could not be more delighted for our clients, the Bylong Valley Protection Alliance, who have dedicated years of their lives to challenging this destructive and inappropriate coal mine proposal."

In 2018 the NSW Department of Planning and Environment referred KEPCO's mine application to the Independent Planning Commission (IPC) who refused it in September 2019 - citing concerns around groundwater impacts, soil rehabilitation, natural and heritage values and greenhouse emissions.

Attempting to have the decision overturned, KEPCO went to the New South Wales Land and Environment Court in August 2020 and in December the court upheld the IPC decision.

KEPCO then took the decision to the NSW Court of Appeal in March the following year and was rejected unanimously by three judges in September 2021. This led to KEPCO seeking leave to appeal to the High Court of Australia in October 2021.

IN OTHER NEWS:

"KEPCO believes that the NSW Court of Appeal made errors in its decision and has filed a special leave application to the High Court seeking proper legal interpretation," the company said in a statement last year.

Following the High Court's decision on Thursday, KEPCO said they are "disappointed with the High Court's decision to dismiss the special leave application" and "will now take some time to consider its next steps".

The EDO say the decision means KEPCO "has exhausted all available legal avenues".

"The IPC's decision to refuse this mine was sound. It was based on the evidence and the science, including evidence about the problematical greenhouse gas emissions," Rana Koroglu said.

"The decision was tested to its limits and in every appeal the IPC's decision has been upheld.

"It means the IPC can be assured that an evidence-based decision to reject these kinds of destructive fossil fuel projects in the future is legally supported."

For Lock the Gate Alliance's Nic Clyde, Thursday's decision is an opportunity for the NSW government and KEPCO to come to the negotiating table.

"The project KEPCO wanted could not be more dead in the water," he said.

"The question now for KEPCO is that they still hold the land and still have a lease for coal in the valley. The government in NSW has never officially taken Bylong off the list for areas allowed to be mined.

"It's time for KEPCO to let residents get back to living their lives without the threat of a coal mine hanging over their land. That may mean government needs to step in and do some negotiating like they did in the Liverpool Plains."

Phillip Kennedy said while he hopes KEPCO will sell the land back, ideally it will go to local "mum and dad farmers".

"It'd be great if they decided to sell it back to the farmers - but as they bought it. To local farms bit by bit rather than as a job lot to a foreign national."