Crown prosecutor Margaret Cunneen has criticised the powers of the Independent Commission Against Corruption as "extraordinarily draconian" after a decision by NSW authorities not to charge her over allegations she had tried to pervert the course of justice.

Two days after the decision to not prosecute her over allegations she advised her son Stephen Wyllie's girlfriend, Sophia Tilley, to fake chest pains to avoid a breath test after a car accident she was involved in last year, Ms Cunneen blasted ICAC for the quality of its investigation.

Crown prosecutor Margaret Cunneen. Credit:Daniel Munoz

"Even the most elementary investigative step of speaking to any of the kind bystanders who helped Sophia from the wreckage would have revealed that she was being treated in the ambulance before the police arrived," Ms Cunneen told Fairfax Media.