Resource consent has been granted for the Kapiti Expressway, north of Wellington.
In a draft decision the Transport Agency has been given the green light to build the 16km Mackays Crossing to Pekapeka motorway.
The $630 million project will take up to five years to complete, involve the construction of at least 18 bridges and employ up to 500 workers at various times.
The project also involves significant land rezoning and the Crown will have to acquire all or part of 84 private properties.
In its written decision released on Monday, the independentboard of inquiry says the new four-lane expressway will introduce a safer route for drivers on State Highway 1.
The 326-page decision says some rivers, streams and wetlands will be affected, but the proposed mitigation measures are appropriate and will be positive for the freshwater ecology of the area.