The New South Wales Independent Commission Against Corruption (ICAC) has lost a High Court bid to investigate crown prosecutor Margaret Cunneen.

The ICAC went to the High Court after it was blocked from investigating claims Ms Cunneen had tried to pervert the course of justice by coaching a witness.

Ms Cunneen was accused of advising her son's girlfriend Sophia Tilley to fake chest pains to avoid a breath test after a car accident in May last year.

Ms Cunneen rejected the claims against her, which she said were part of a malicious complaint.

The prosecutor's lawyers told the court in any case such an inquiry was beyond the scope of ICAC.

The NSW Court of Appeal agreed, prompting today's hearing in the High Court.

Today, the High Court found the commission did not have the power to investigate because the allegations did not constitute corrupt conduct under the ICAC Act.

The commission has been ordered to pay costs.

After the decision was handed down, Ms Cunneen told the ABC she was "glad that justice was finally done".

"No family should ever be put through this again," she said.

The ruling has helped define the scope of the commission's powers.

During the hearing Ms Cunneen's counsel David Jackson said ICAC should direct its attention to serious and systemic corrupt conduct.

Mr Jackson said the ICAC Act did not include provisions wide enough to take in the allegations against Ms Cunneen.

A friend of Ms Cunneen's, Sydney Barrister Jeffrey Phillips SC, said a decision against the ICAC had been coming for a while.

"A case like this, clearly showed in sharp relief, that the ICAC was going into areas that were never part of its function," he said.

"It interpreted corrupt conduct in a way which could've meant any behaviour of any public official or anyone dealing with a public official, which had nothing to do with that public official's task."

NSW Acting Premier Troy Grant said he would discuss the outcome with Premier Mike Baird upon his return to Sydney.

"The Government will review the High Court's decision and take all appropriate advice on any action required," he said.

A key focus of the hearing was to review parliamentary speeches about the intent of those who put the ICAC Act in place.

The commission had warned if it lost its bid, key investigations could be in jeopardy, although it is not yet known if this will be the case.

The ICAC reports on inquiries including the Obeid-related Australian Water Holdings and NSW Liberal Party.

Both inquiries were on hold until the case against Ms Cunneen was resolved.

In a statement the ICAC said it was currently considering the decision of the High Court and would be make a public statement in due course.