The state's highest court has rejected a bid by mining mogul Travers Duncan and three of his associates to overturn corrupt conduct findings made against them over a multimillion-dollar coal deal.

Mr Duncan, lawyers-turned-businessmen John McGuigan and John Atkinson, and investment banker Richard Poole were the founding investors in Cascade Coal, a company seeking to exploit a coal tenement in the Bylong Valley near Mudgee.

Mining mogul Travers Duncan outside the ICAC in 2012. Credit:Rob Homer

After an explosive public inquiry that also made corruption findings against former NSW Labor ministers, the Independent Commission Against Corruption made adverse findings against the businessmen and Cascade Coal in 2013.

The businessmen lost a Supreme Court bid in July 2014 to have the findings overturned.