The $930m, 27km Transmission Gully route has gained final approval.

The project could be finished by 2021, with a road cut through the hills from Linden to rejoin State Highway 1 north of Paekakariki.

The Environmental Protection Agency this morning released the finding of a board of inquiry meaning the only thing that can stop the road now is an appeal to the High Court on a point of law.

Estimates state it will shave 10 minutes off Kapiti peak-hour commuters' trips to and from Wellington.

It will be one of New Zealand's biggest infrastructure projects and the Wellington region's biggest roading project since the Wellington motorway extension in the late 1970s.

In May, the board of inquiry gave provisional consent for the road but sought feedback ahead of today's final decision.

In its May finding, the board said the existing SH1 route through Paremata and Pukerua Bay was inadequate, congested, severed coastal communities and could close in an earthquake or tsunami.

The new route would ''rectify those inadequacies by providing a new four-lane route which will avoid congestion, reduce travel times and achieve consistency in travel times''.

''The new route will be safer than the coastal route.''

However, the decision also listed environmental fallout, including an increase in sediment to Porirua Harbour, destruction of some indigenous growth, and the destruction, modification, or diversion of 10km of streams.

Sediment from construction and the effects on streams, in particular the nationally significant Pauatahanui Inlet, was of paramount importance, the board said.

Just how much sediment could potentially flush into the inlet was a contentious issue and one that experts could not agree on.

Based on additional information, the board concluded that during construction an additional 3024 tonnes of sediment would enter Porirua Harbour, which is between 4 and 5 per cent of the sediment that would flush into it naturally.

There would be about 6 million cubic metres of earthworks, making it the largest earthworks project ever undertaken in the Wellington region.

The draft report said consent should be issued for the main route and Porirua link roads, but it came with more than 100 pages of conditions.

Transpower, one of three applicants for consent, will need to move pylons which hold wires going to Kapiti and Pauatahanui.

It will have to move some power infrastructure but a spokeswoman said there would be no power outages as a result.

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