The NSW Government amended the state environmental planning policy for mining today after the Bylong proposal's 2014 Gateway Certificate lapsed in April on the eve of a final decision on the project between Denman and Mudgee.

The move is in line with Bylong owner KEPCO's submission that Gateway Certificates, which provide an initial scientific assessment of mine proposals, remain current until a final determination.

The KEPCO position was at odds with legal advice from the Environment Defenders Office, and the preliminary view of the Independent Planning Commission, that the commission could not make a final decision on the mine "in the absence of a Gateway Certificate that is current at the time of the determination".

The Korean Government-backed mining company knew the certificate had expired at least six weeks before a NSW planning commission raised it as an issue, documents showed.

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.

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.

Lock the Gate Alliance, which raised the lapsed Gateway Certificate with the Independent Planning Commission in July, said the new amendments failed to give certainty to the state's farmers by leaving strategic agricultural lands available for mining.

The Bylong coal mine will remove 13 per cent of mapped strategic agricultural land in the valley.

"Minister Stokes has failed again to address the underlying cause of mining conflict in NSW by clearing the way for the Bylong mine to be approved and failing to establish basic protections for the most fertile soils in the state," Lock the Gate spokesperson Georgina Woods said.

"Strategic agricultural land is rare and should be afforded the highest protection. Instead, in NSW, it's available for open cut coal mining.

"The fate of the precious Bylong Valley is in the hands of the Independent Planning Commission, which could make its decision any hour now."

The gazetted changes announced today create a 2.5 kilometre buffer around the village of Bulga. The buffer was welcomed but came four years after the NSW Government committed to do so, Ms Woods said.

In a statement on Friday Mr Stokes said there were "no winners when a project is subject to unnecessary delays".

"I am determined to bring development applications to a decision as soon as possible. Without this change, communities, miners and taxpayers could face ongoing delays and costs," he said.

"This amendment provides procedural certainty to ensure that long-running mining applications can be determined, one way or another, without impacting the rigorous merit-based assessment of projects."