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.

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.

But one of their associates, RAMS Home Loans founder John Kinghorn, was successful in that case because the finding against him was made on a narrower basis that the court ruled was invalid.

Mr Kinghorn became the first person in more than 20 years to be cleared of corrupt conduct in NSW.

The Court of Appeal ruled on Wednesday that the findings against Cascade Coal, Mr Duncan and his associates were valid.

Chief Justice Tom Bathurst, dissenting in part, would have quashed the corruption findings against Mr Atkinson and Mr Poole. But he agreed on most points with Court of Appeal President Margaret Beazley and Justice John Basten.

All three judges rejected a challenge to the ICAC's jurisdiction to recommend Cascade Coal's exploration licence be torn up because it was "tainted by corruption".

The court also found that the basis on which a corrupt conduct finding was made against Mr Kinghorn was in fact valid.

But because the ICAC had already agreed to discontinue the case against Mr Kinghorn, no finding will be reinstated against him.

The businessmen have 28 days to file any application for special leave to the High Court.