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, 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.

His lawyers argue the laws are an "impermissible direction" to the courts not to overturn corruption findings in light of the Cunneen decision.

But the Attorneys-General of Victoria, Queensland, South Australia and Western Australia have backed NSW in the dispute.

"The impartiality of the courts is not undermined by the courts applying a retrospective alteration to the substantive law," the Victorian Attorney-General says in submissions to the High Court.

"The effect of [Mr Duncan's] argument is that the state could never pass a retrospective law, which is contrary to well-settled authority and practice."

Mr Duncan and three of his associates were found to have acted corruptly by concealing the involvement of the Obeids in a mining venture.

The Cunneen ruling would have invalidated those findings, but the retrospective laws validated anything done by the commission before the ruling on April 15.

Mr Duncan's lawyers argue the "historical fact" of the invalidity of the corruption findings cannot be "altered or erased" by the retrospective laws.

The High Court will hear the case on August 5. A panel led by former High Court chief justice Murray Gleeson, QC, and Sydney silk Bruce McClintock will deliver a report to the government on Friday about the future direction of the watchdog.

The review was commissioned by the Baird government after the Cunneen decision and has major implications for current and future inquiries, including findings which may be made in its investigations into political donations and the Obeid-linked company Australian Water Holdings.