The Baird government is trying to keep secret key documents related to the $13 billion WestConnex motorway project, claiming they are "privileged" and should be only available to MPs who are forbidden from revealing their contents.

Plans for the WestConnex, which is largely to cut under the inner west, are in a state of flux. The government has pushed back the date for detailed plans to be released for the contentious M4 East section of the project under Parramatta Road well into next year.

WestConnex: An artist's impression, looking towards the south-west.

The boxes of documents that have been released previously to the public reveal disagreements among bureaucrats and consultants preparing the business case for the motorway.

The move to declare a further tranche of documents as privileged is being challenged by the Greens in the upper house, leading to the appointment of an independent arbiter, Keith Mason, QC, a former president of the NSW Court of Appeal, to rule on the matter.