Margaret Cunneen has done what dozens of politicians and targets of corruption inquiries have dreamed of doing in NSW: clipped the wings of the state's corruption watchdog.

The majority High Court ruling that the Independent Commission Against Corruption has no power to investigate the high-profile Crown prosecutor is likely to result in a series of corruption findings being overturned and prospective inquiries killed off.

It's a good result for the 56-year-old silk, and no doubt her supporters will be cheering. But the people of NSW have little cause to pop the champagne corks.

At the heart of the Cunneen case was a battle over the definition of "corrupt conduct" in the ICAC Act.

As Sydney silk Bruce McClintock observed in an independent review of the Act in 2005: "The definition of corrupt conduct is crucial to the effectiveness of ICAC as it defines the scope of ICAC's investigatory jurisdiction."

The court has now given a narrow interpretation to one form of "corrupt conduct" defined in the Act.

This form of corruption involves any person engaging in conduct that "could adversely affect" the exercise of a public official's functions, and which could constitute one of a number of specified criminal offences.

The McClintock Review said this was a kind of "indirect corruption" and would catch cases in which a person allegedly deceived a public official to bring about a corrupt result.

But the High Court has now ruled the definition does not extend so far. For a person to "adversely affect" the exercise of a public official's functions, the probity or honesty of the public official must be adversely affected. It is not enough that they were deceived.

Accordingly, the allegation (denied by Cunneen) that the silk advised her eldest son's girlfriend to feign chest pains to avoid a police breath test was not corruption, because the police officer did not act dishonestly.

The ruling means a number of previous corruption findings – made under the same provision of the Act – are likely to be overturned.

Mining mogul Travers Duncan and others were found to have acted corruptly by concealing from NSW officials the interest of the Obeid family in a coal tenement.

Similarly, former CFMEU boss John Maitland and others were found to have acted corruptly by making or agreeing to misleading statements being made to bureaucrats about a Doyles Creek "training mine".

The ICAC will also have to radically revise the findings it makes in its explosive inquiries into Obeid-linked company Australian Water Holdings and Liberal Party donations, both of which involved claims innocent bureaucrats may have been hoodwinked.

As Justice Stephen Gageler notes in his dissenting judgment, the High Court was presented with a choice "between two extreme consequences".

On the one hand, applying the broader interpretation of the ICAC's powers, the commission has the "power to investigate an isolated case of a witness telling a lie to a police officer" and potentially a raft of other criminal offences.

On the other hand, applying the narrower interpretation, the ICAC has "no power to investigate ... state-wide endemic collusion among tenderers in tendering for government contracts".

It is true many of these allegations could be investigated by the police. But the reality is allegations of this kind may never come to light without the corruption watchdog.

As Justice Gageler put it, the question was not about the "propriety or prudence" of ICAC choosing to investigate Cunneen, but whether it had the power to do so.

We may never know if the ICAC chose its target wisely because the evidence it had gathered will not be presented at a public inquiry. The Cunneen investigation, now thwarted, has been the trigger to cruel the commission's powers – and it is unlikely Parliament will be persuaded to give them back.