The High Court of Australia has clipped the wings of the high-flying NSW Independent Commission Against Corruption, the nation’s most successful corruption-fighting watchdog.

By a majority verdict, the High Court today ruled that ICAC did not have the powers to investigate deputy senior Crown prosecutor Margaret Cunneen SC for allegedly perverting the course of justice.

ICAC had appealed to the High Court against a NSW Court of Appeal decision to shut down its public inquiry into the so-called “Cunneen affair”, which centred on allegations she had advised her eldest son’s girlfriend to use “chest pains” as an excuse to avoid a police breath test following a traffic accident.

ICAC argued that the allegation warranted a full public inquiry, while Cunneen and her barrister, Arthur Moses SC, said the watchdog was acting beyond its legislative reach.

As expected, the High Court dismissed ICAC’s appeal and found that the Cunneen case did not amount to “corrupt conduct” within the meaning of the ICAC Act.

The decision will prompt a review of ICAC by the Baird government and its new Attorney-General Gabrielle Upton, a former merchant banker and the state’s first female AG.

Backed by a collection of spivs and urgers from the Liberal, National and Labor parties, the review will attempt to curtail ICAC’s “name and shame” public hearings and introduce identity exemptions for high-profile targets.

As legal commentator Richard Ackland told Crikey today: “The High Court has narrowed the scope for investigating ‘corruption’. It has dreadful implications for past and pending findings.”

By choosing to go after Cunneen, the watchdog has inadvertently opened the floodgates to its own ruin.

Cunneen, who resigned from her senior position in the criminal justice system last year, is expected to resume her duties shortly, but her future legal career will be the subject of ongoing controversy.

In a second, simultaneously released judgment, the High Court dismissed an appeal by a group of NSW coal barons challenging ICAC’s right to strike out their exploration licences.

After extensive public hearings in 2014, ICAC recommended that NuCoal, Cascade Coal and its former director, Travers Duncan, be stripped of their licences because the approval process had been tainted by corruption.

The coal companies, which held licences in the Hunter River region at Doyles Creek, Mount Penny and Glendon Brook, argued that the withdrawal of their licences was unconstitutional, a proposition that the High Court has rejected.

The High Court’s verdict clears the way for ICAC to publish reports into the coal-licensing scandal, which involved members of former NSW MLC Eddie Obeid’s family and the former mineral resources minister Ian Macdonald.

Macdonald, the long-time leader of the “hard Left” faction, was sacked by premier Nathan Rees in 2009 but restored to the cabinet by his successor, premier Kristina Keneally.

When he resigned from Parliament in 2010, Macdonald’s vacant position in the upper house fell to Luke Foley, the current Opposition Leader, who shifted to the lower house seat of Auburn at last month’s state election.

Protection of ICAC’s ongoing role now depends on bipartisan commitments by all NSW parties to support the watchdog’s funding, staffing and vigorous anti-corruption crusading.