The states have intervened to support NSW in a High Court battle over the validity of laws shoring up the powers of the Independent Commission Against Corruption, as the watchdog faces a potential shakeup of its powers.

The NSW Parliament passed urgent laws in May to validate past corruption findings and investigative activities of the ICAC, which were jeopardised by a High Court ruling that it had no power to investigate Crown Prosecutor Margaret Cunneen, SC.

Mining mogul Travers Duncan. Credit:Michele Mossop

Mining mogul Travers Duncan, who was ensnared in an inquiry into the Obeid family's coal-rich property, is challenging the retrospective laws on the basis they usurp the powers given to the courts under the Constitution.