Cunneen probe will leave collateral damage

Top prosecutor Margaret Cunneen's son and his girlfriend will become collateral damage if a public corruption inquiry into the top silk's conduct goes ahead, the court of appeal has heard.

Ms Cunneen is appealing a decision by the NSW Supreme Court to allow an ICAC probe into allegations she perverted the course of justice following a car crash involving her son's girlfriend, Sophia Tilley.

The Independent Commission Against Corruption is investigating claims Ms Cunneen coached Ms Tilley to complain of chest pains following the May accident so police could not obtain her blood alcohol level.

Ms Cunneen's barrister, Arthur Moses SC, on Tuesday asked the court to consider the "collateral damage" of hauling Ms Tilley and Ms Cunneen's eldest son, Stephen Wyllie, before the commission.

"Everyone is focusing on Ms Cunneen, but we're also talking about two young people who through their relationship with Ms Cunneen have lost their rights," Mr Moses said.

"When one considers their rights ... then the miscarriage of the decision to investigate is clearly revealed."

A person had no right to silence when brought before ICAC, he said.

Mr Moses said using ICAC to investigate an alleged criminal act - instead of corruption in public office - set a dangerous precedent.

If Ms Cunneen were to be called in front of ICAC for an alleged criminal matter, it could mean any citizen could be called before the inquiry and lose their right to silence, Mr Moses said.

"It's using a sledgehammer to crack a nut," he said.

"Where does it stop?"

But the barrister for ICAC, Jeremy Kirk SC, said the inquiry would seek to establish what happened.

"It's not a prosecution, it's an investigation," he said.

"There is a clear allegation all three tried to pervert the course of justice."

Much of Tuesday's appeal hearing focused on whether Ms Cunneen's alleged conduct falls under ICAC's jurisdiction.

The corruption watchdog has put on hold its public inquiry until the appeal is determined.

ICAC cannot find that a crime has been committed, but can recommend further action to prosecutors if corruption is found.

The judges have reserved their decision.