An Environment Protection Authority board of inquiry has heard evidence from the three applicants wanting resource consent to build a four-lane highway known as Transmission Gully, north of Wellington.
The Transport Agency, Porirua City Council and Transpower want permission to build the 27km-long road stretching from MacKays Crossing south to Linden.
The agency and council's lawyer, John Hassan, opened the inquiry on Monday saying the route would provide an alternative to State Highway 1 along the coast.
Mr Hassan would bring many benefits, including reducing travel times, improving road safety and providing better resilience to earthquakes.
He also said construction would have an impact on the environment that would have to be mitigated with measures such as restoring streams and replanting more than 500ha.
Mr Hassan argued that plans for the route and the way it would be constructed were well refined after decades of planning and submitted it would take six years to build.
Transpower would have to relocate part of a transmission line that runs through the gully.
The hearing is expected to last a month and the independent board of five will make a decision by June this year.