The High Court is set to have the final say about whether the Independent Commission Against Corruption can investigate Crown prosecutor Margaret Cunneen over allegations she perverted the course of justice.

Chief Justice Robert French on Friday referred the commission's application for special leave to appeal a ruling shutting down its investigation into Ms Cunneen, SC, to the full court of the High Court, which will hear the appeal in March.

Ms Cunneen has denied allegations she told her son's girlfriend to feign having chest pains to avoid a blood-alcohol test following a car accident.

Counsel for the ICAC, Jeremy Kirk, SC, told the court that the 2-1 majority decision of the NSW Court of Appeal had "knock-on effects for a wide range of people and a wide range of inquiries" and an appeal should be heard as soon as possible.

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But Arthur Moses, SC, for Ms Cunneen, said "sweeping statements" by the commission about how "in essence, its whole being was affected" by the Cunneen decision were "an overkill".

The ICAC was due to hand down reports in its high-profile inquiries into Obeid-linked company Australian Water Holdings and Liberal Party donations in late January.

It now says it will not complete the reports until the Cunneen proceedings are resolved, because the ruling "fundamentally alters" the basis of its powers to conduct those investigations.

The ruling is also expected to be relied upon by people challenging the ICAC's findings in past inquiries, including a group of businessmen who were found to have acted corruptly by concealing the involvement of the Obeid family in a coal tenement.

Mr Moses said it was "not immediately apparent" why the ICAC was "putting its pen down" in the Australian Water and political donations inquiries, which were "completely unrelated to this matter".

He added that a pending High Court challenge by property developer and former Newcastle mayor Jeff McCloy to the NSW ban on political donations from property developers would also affect the donations inquiry.

Also on Friday, the Federal Court lifted a suppression order on fresh documents handed over to the ICAC about Australian Water.

The documents were discovered as part of Sydney Water's case against Australian Water to claw back almost $1 million in costs it claims were improperly billed to it by Australian Water, including for limousines, air fares and gifts.

During its public inquiry, the ICAC heard a subsidiary of Australian Water withheld from Sydney Water details of its expenses, claiming it could not get the information. 

The new document, referred to the ICAC by Sydney Water, suggests this was not the case.

A spokesman for Sydney Water said it believed the material "should be drawn to the attention of the ICAC" because it was relevant to Australian Water's obligations to provide details of costs charged to the utility.

"It is now a matter for the ICAC to consider and determine what, if any, course of action it may take in relation to the material," the spokesman said.