It comes only weeks after Korean energy company KEPCO, in a strongly-worded submission, said it had spent $750 million on the project so far after it was "encouraged" to do so by the NSW Government and Department of Planning.

The Independent Planning Commission has given KEPCO until Friday to say if it disputes the commission's view it cannot approve the mine in the absence of a current gateway certificate.

The Bylong mine gateway certificate expired on April 15.

Lock the Gate Alliance questioned the commission about the expired certificate last week before the commission wrote to KEPCO on Thursday seeking its response.

In a statement on its website today the commission said it asked KEPCO to either make a submission on the implications of the expired gateway certificate for the development by Friday or advise the commission if it intended to apply for a fresh certificate.

"In its letter to the applicant, dated 18 July 2019, the commission advised it is of the preliminary view that it cannot, as consent authority in respect of the project, approve KEPCO's state significant development application 'in the absence of a Gateway Certificate that is current at the time of the determination'," the commission said.

The Bylong proposal was one of the first in NSW to be assessed under the gateway certificate process that was introduced by the NSW Government to manage increasingly controversial clashes between mining and agriculture.

But the conditional certificate issued to KEPCO increased criticism by community, environment and agriculture groups, including NSW Farmers, after the Bylong proposal was found deficient in 12 of 13 assessment areas, including impacts on soil, fragmentation of land uses, groundwater and subsidence.

The gateway certificate panel issued a conditional certificate because its only other option was to issue an unconditional certificate.

A Planning Assessment Commission panel in 2017 issued a scathing assessment of KEPCO's proposal for an open cut and underground mine to produce 120 million tonnes of coal, and warned the Department of Planning it would have to carefully and cautiously consider the project's significant risks and unanswered questions before making a final decision.

An amended KEPCO proposal to mine up to 6.5 million tonnes of coal per year over 25 years for use in the domestic Korean energy market is currently before the Independent Planning Commission.

In 2014 NSW Farmers president Fiona Simson said the Bylong gateway decision "realised our worst fears in relation to the operation of the gateway panel".

"It's obvious this system is flawed as projects sail through with no ability for the panel to recommend that a project not go ahead. We've been highly vocal in our criticism of this process for a while, but it should now be obvious to government it needs strengthening," Ms Simson said.

The expired gateway certificate is just the latest problem for the controversial Bylong project between Denman and Mudgee, after the Independent Planning Commission advised on May 1 that it would not accept any further submissions as a final decision was "now pending", only to call for fresh submissions on June 27 because of new issues raised in a heritage report.

Lock the Gate Alliance spokesperson Georgina Woods said the expired gateway certificate was a lifeline for the threatened farming district and its residents and an opportunity for the NSW Government to intervene against the mine.

"The lapsing of this certificate puts the spotlight back on the terrible impacts of the Bylong coal mine on agricultural land and water resources, including the Bylong River," Ms Woods said.

"We need the NSW Government to ensure that this damaging coal mine does not go ahead. The best way to do that is for the NSW Government to make strategic agricultural land and industry clusters off-limits to mining as they should be," Ms Woods said.

"It is absolutely the wrong place and the wrong time for a new coal mine."

A KEPCO spokesperson said the company "disagrees with the preliminary view of the Independent Planning Commission in regard to the gateway certificate and will provide a comprehensive response to them for consideration".