A legal challenge is being mounted over the Transmission Gully roading proposal north of Wellington.
In September the Government referred the nearly $1 billion project to an independent board of inquiry to fast-track its consenting process.
The Transport Agency, Porirua City Council and Transpower are jointly seeking approval to build the 27km inland alternative to State Highway One from Linden to Mackay's Crossing.
But the Rational Transport Society is challenging the legality of the development on environmental grounds.
It says about 10km of fresh water streams will be affected by the project and it does not believe the board of inquiry has the legal authority to approve the development because of its impact on waterways.