It warns the result "has the potential to involve the State of NSW and the commission in costly and protracted litigation involving persons who have been the subject of corrupt conduct findings" under the section of the ICAC Act at the heart of the Cunneen case.

It will also "affect current litigation involving such findings", an apparent reference to an appeal by mining mogul Travers Duncan and others against corruption findings made against them following an investigation into a coal tenement over the Obeid family's Bylong Valley farm.

The ICAC said the decision meant the commission "will be unable to investigate or report on several current operations", which had not been revealed, and will "severely restrict" its ability to report on inquiries into Obeid-linked company Australian Water Holdings and Liberal Party fundraising activities.

It also "has the potential to call into question the prosecutions and convictions of persons where evidence against them was obtained during commission investigations" that were based on the section of the ICAC Act interpreted by the High Court.

"In the commission's view, the narrow construction adopted by the majority in the High Court is contrary to the legislative intention evidenced by the second reading speech when the ICAC Act was first introduced, the analysis of the section in the report of the McClintock review of the ICAC Act [in 2005] and the ordinary meaning of the words used in the section," the ICAC said.