Transport Minister Simeon Brown said the improvements would result in "significant travel time savings". Photo: RNZ / Angus Dreaver
The government has locked in its preferred plan to lower congestion and create a faster route to the airport, including two new tunnels and grade separation at the Basin Reserve.
The first new tunnel would run under Mount Victoria, at 0.7 km long, parallel to the existing tunnel, and the second would run parallel to the existing Terrace Tunnel, measuring 0.5 km.
The government previously floated the idea of a 4km "Long Tunnel" under the city.
The changes, to be carried out by Waka Kotahi NZ Transport Agency, are part of the programme called Roads of National Significance (RoNS), and are aimed at reducing Wellington's gridlock and supporting economic growth, said Transport Minister Simeon Brown.
Modelling had estimated there would be a 30 percent reduction in southbound traffic across the Harbour Quays once the second Mt Victoria tunnel, Basin Reserve upgrade, and the duplicate Terrace Tunnel were all completed.
"Without improvements to this critical route, within ten years the region could be looking at travel time increases of up to 50 percent between Ngauranga and the airport," Brown said.
It would also involve grade separation - a method of reducing disruption by passing lanes over one another - of State Highway 1 and local roads through the Basin Reserve Upgrade project, by extending the Arras Tunnel and moving the westbound SH1 traffic to the northern side of the Basin.
Local traffic, including public transport, would pass over SH1 on the western side of the Basin.
"These improvements will result in significant travel time savings during peak times, reducing travel times from the northern suburbs to the CBD, hospital, and airport by approximately 10 minutes," Brown said.
"Commuters in the capital will see a significant benefit in time savings from these upgrades. Those travelling on the number one bus between Island Bay and the Railway Station are forecast to save nine minutes during morning peak times, while those on the number two bus between Miramar and the CBD will notice a saving of 11 minutes on their journeys."
The project would be consented as a single corridor, with the project delivered in stages.
An investment case should be brought back to the NZTA Board in mid-2025.