The head of the NSW Independent Commission Against Corruption (ICAC) has refused to answer a parliamentary committee's questions about why the corruption watchdog investigated top crown prosecutor Margaret Cunneen.

Commissioner Megan Latham has appeared before a committee hearing in state parliament as part of a regular review into the ICAC.

She was questioned about the high-profile investigation of Ms Cunneen over allegations she advised her son's girlfriend about how to avoid a breath test after a car accident.

The High Court ruled the case was outside of the ICAC's powers and Ms Cunneen has criticised the ICAC, saying it needs an overhaul.

Ms Latham said the ICAC carefully chooses which cases are in the public interest to pursue but said she did not have to explain them to the committee.

"I'm not trying to be rude, I'm simply trying to point out that section 64 does not allow this committee to ask these questions," she said.

But the committee's chair, Liberal MP Damien Tudehope, did not accept her argument.

"With respect I disagree, commissioner. I think that's an entirely relevant question," he said.

"Well, we'll have to agree to disagree, I'm sorry," Ms Latham replied.

The High Court ruled that the ICAC did not have the powers to investigate Margaret Cunneen SC. ( AAP: Nikki Short, file photo )

Ms Latham was also grilled about why the ICAC released a press release after the High Court decision, announcing that it was passing the material it had collected in the Cunneen investigation on to the Director of Public Prosecutions (DPP) so it should consider whether charges should be laid against her.

Committee members asked whether Ms Latham or the ICAC had considered that Ms Cunneen would have to step aside from prosecuting a murder trial once the news was made public.

"Again, I don't understand where these questions are going or why they come within the remit of the committee," Ms Latham said.

"But can I just say this: When the commission makes a decision that something is in the public interest to convey publicly, we make that decision regardless of the effect that it might have on that individual.

"If it's in the public interest, it's in the public interest."

The state's solicitor-general has since announced the material had been considered and Ms Cunneen would not be charged.

ICAC inspector says case should not have been pursued

Earlier, the ICAC inspector, former judge David Levine QC, told the committee he believed the watchdog should never have pursued the case against Ms Cunneen.

He said the watchdog should only be dealing with "big-time stuff".

Mr Levine referred to operations Spicer and Credo, which prompted the resignation of former premier Barry O'Farrell and saw 10 Liberal MPs either resign or move to the cross benches.

"Until the motor vehicle accident on May 31, 2014, ICAC had been concerned with major inquiries, major inquiries involving Operation Jasper, Spicer, Credo involving politicians from the premier down," he said.

"The premier resigned from his office, and I think nine others moved to the cross bench? I cannot recall how many, but I know it was a sufficient number to constitute what I perceived to be a disruption of the structure of parliamentary government.

"The fact that that nine, 10, 11 members had to move from one side to the cross benches or whatever, that was big-time stuff."

He said the public expected the ICAC to use its powers for significant cases.

"I can criticise it in terms of how it administers its powers, I can criticise it from a perspective of a sense of proportion that I and I think the public would expect to be exercised by the ICAC in relation to the matters in which it concerns itself," Mr Levine said.