An independent board's green light for the proposed Transmission Gully inland highway linking Wellington with the Kapiti Coast is already drawing flak from the Greens.

The Environmental Protection Authority issued the final decision on Friday of a board of inquiry which approved resource consents and confirmed notices of requirement for the $1 billion, 27km highway.

The route, from Linden to MacKays Crossing, is part of the Wellington Northern Corridor which runs from Levin to Wellington airport.

It is one of seven "roads of national significance" that the government has identified as essential state highways which require upgrading to reduce congestion, improve safety and support economic growth.

But Greens transport spokesperson Julie Anne Genter says the project does not add up when a cost-to-benefit ratio is applied and would harm the environment.

"Transmission Gully is bad for commuters and bad for the environment. It will undermine commuter rail, increase overall traffic in Wellington City and increase greenhouse gas emissions," she said.