The Greens will introduce a bill in State Parliament to give the Independent Commission Against Corruption the power to make findings that have been thrown into doubt by a landmark High Court ruling.

A four-one majority of the High Court ruled on Wednesday that the commission had no power to investigate Crown prosecutor Margaret Cunneen, SC, over allegations of perverting the course of justice.

Greens MP Jamie Parker has immediately moved to rectify the ICAC Act following a High Court decision. Credit:James Alcock

The ruling has paved the way for legal challenges against past corruption findings in other inquiries. It has also cast doubt on the commission's ability to make some findings in its inquiries into Obeid-linked company Australian Water Holdings and Liberal Party fundraising.

Greens MP Jamie Parker said the party consulted legislative drafters in the Parliamentary Counsel's Office on Thursday to devise a bill to shore up the breadth of ICAC's capacity to investigate corruption and to "make secure findings that they've made in the past".