Documents show KEPCO received legal advice on June 2 about the expired Gateway Certificate but appears not to have advised the NSW Independent Planning Commission, which first raised it with KEPCO in a letter on July 18 as it prepared to make a final decision on the mine project.

KEPCO declined to respond to questions about the legal advice, when it became aware the five-year April, 2014 Gateway Certificate had expired and why it appeared to have failed to advise the IPC, after condemning the IPC for raising it "now and at this very late and critical stage of the assessment process".

"This issue certainly should have been raised with KEPCO sooner and not three months after the date on which the commission asserts the Gateway Certificate 'expired'," the company said in a letter on August 5 after the IPC re-opened submissions from the public on the issue.

The expired Gateway Certificate - granted after a scientific panel consideration of a proposal's impact on agricultural areas - has thrown the controversial Bylong mine between Denman and Mudgee into doubt nearly a decade after KEPCO bought the exploration licence from Anglo American.

It followed questions about the expired Gateway Certificate from environment group Lock the Gate in mid July, which prompted the IPC to write to KEPCO on July 18 questioning whether it could make a final decision without a current certificate, as required under a clause of the Environmental Planning and Assessment Regulation.

The Independent Planning Commission re-opened submissions nearly two months after advising the public on May 1, via its website, that a final decision was imminent. The new public submission period ended on August 9.

In a letter to the IPC on August 5 KEPCO strongly criticised the commission and referred to legal advice it provided, dated June 2, saying the certificate did not need to be current at the time of determination and "there is no lawful reason why the IPC cannot determine" the application.

"The only reason such a potential issue has arisen is the significant and unreasonable time the IPC has taken (10 months and counting) to determine the application following referral of the project to the IPC on 4 October, 2018," KEPCO said.

"Since December, 2018 KEPCO or its representatives have made a significant number of telephone calls to the IPC regarding matters relating to the determination of the project, including seeking an indication when the determination was likely to occur and whether additional information was required.

"At no time did IPC representatives suggest that the currency of the Gateway Certificate was an issue that could potentially impact the IPC's determination. During these contacts there was ample opportunity to raise the matter."

KEPCO did not respond to Newcastle Herald questions about the June 2 legal advice and what action it took after obtaining it.

The legal advice included that if the "proper construction" of the regulation was that a development application had to be accompanied by a current Gateway Certificate, the application "cannot validly be determined".

In June KEPCO alleged the NSW Government and Department of Planning "encouraged" a controversial $115 million land buy-up in Bylong Valley to reduce public criticism, as part of a $750 million commitment it has made to the project.

KEPCO was "encouraged to acquire all of the land to be either directly or indirectly affected by the development", it said in a June 27 submission to the IPC.