A landmark High Court ruling will decide the future direction of the Independent Commission Against Corruption and the validity of a raft of findings against high-profile targets of its investigations.

The court will deliver its decision on Wednesday in the legal battle between the ICAC and Crown prosecutor Margaret Cunneen, SC, over a proposed inquiry into allegations she perverted the course of justice.

The ICAC is in the High Court seeking a ruling which allows it to investigate Margaret Cunneen.

Inquiries that may be affected by the ruling include the investigation into Obeid-linked company Australian Water Holdings, in which findings are yet to be delivered, and the inquiry into a coal tenement created "smack bang on top" of the Obeid family's farm.

At the heart of the High Court battle is the meaning of "corrupt conduct" as defined in the ICAC Act. The Act contains multiple definitions of corruption but only one, involving the conduct of private citizens, is being examined.