Wellington's Mt Victoria tunnel is going green as part of an upgrade that has been underway since 2012.

The vintage tunnel, which was built in 1931, will be painted light green inside with light grey at the entrance, the New Zealand Transport Agency said.

The agency's Wellington highways manager Neil Walker said the light green colour scheme would enhance the tunnel by making it a thoroughfare Wellingtonians could be proud of.

"This is more than just a road tunnel - it's an iconic gateway to the capital."

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It would also enhance driver safety, he said.

The colour was assessed by safety and design experts, who aimed to find a colour that would limit driver distraction and give the tunnel a calming appearance, Mr Walker said.

The exact shade of green would be chosen after sampling onsite, he said.

"Light green has been proposed for the ceiling and part-way down the sides of the tunnel.

"For drivers, the green is replaced by calm black 30 metres in from either side. Lower walls and panels will be soft white throughout."

The colour scheme would carry through into the pedestrian and cyclist area, he said.

The 84-year-old tunnel, which is 623m long, has been upgraded since 2012.

The current work is part of the second stage of the tunnel's three-stage upgrade.

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There is light at the end of the tunnel, as the upgrade is expected to be complete in mid-2016.

Wellington Mayor Celia Wade-Brown said the old tunnel made the approach into the city rather dreary, and so she welcomed the move to greener pastures.

"This freshen-up is long overdue and the new colours will make the drive, cycle or walk, through the tunnel a more pleasant experience."

The third stage of the project will involve creating a new two-lane tunnel for eastbound traffic and reconfiguring the current tunnel for west-bound traffic.

Construction on the second tunnel was scheduled to begin in 2018, but is now more likely to start in 2020 because it was delayed by the proposed Basin Reserve flyover, rejected today.

Tunnel Updates:

• A new communications system, including emergency management

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• Lighting replaced with brighter, more efficient LED lights

• Pedestrian walkway strengthened

• Entrance exterior slope strengthened and concrete surfacing improved

• New wall panels replacing outdated panels