Construction of the Victoria Park tunnel began yesterday.
The project will tackle congestion on one Auckland's busiest stretches of motorway.
It is the final stage of the NZ Transport Agency's Central Motorway Junction improvements and will take about three years to complete.
The 440m tunnel beneath Victoria Park will carrythree lanes of northbound traffic.
It will improve pedestrian access over the motorway to the harbour and familiar sites, such as the Birdcage Tavern, will be protected.
Work began yesterday on widening the southbound lanes between the park and the bridge.
The motorway project is the first of the Government's seven "roads of national significance" to start.
Other improvements include better motorway access to the port, smoother connections between the Southern and Northwestern motorways and replacement of the Newmarket Viaduct.
This will convert the viaduct to four southbound lanes and increase the capacity of the St Mary's Bay section to five lanes in each direction.
A southbound bus shoulder lane and a northbound bus priority lane are also planned.
Transport Minister Steven Joyce yesterday turned the first sod at the beginning of the tunnel work.
The Transport Agency's director for Auckland and Northland, Wayne McDonald, said:
"The project will also provide a boost for public transport in Auckland by unlocking the full benefits of the North Shore busway, removing the current pinch point at St Mary's Bay."
Mr McDonald said that when the project was completed, southbound motorists would be able to stay in the same lane from the Harbour Bridge to the city, the Southern Motorway or the Northern Motorway, removing the need for merging and lane-changing through St Mary's Bay.
The motorway speed limit through St Mary's Bay has been reduced to 70km/h because of the work.
Big job
Length of project: 2.2km.
Length of tunnel: 440m.
Estimated cost: about $400m.
Work force expected to peak at 450.
Motorway tunnel will be urban Auckland's first, and will have three northbound lanes.
Up to 200,000 vehicles a day now use the Victoria Park viaduct.
A bridge for pedestrians and cyclists will be built over the motorway linking St Mary's Bay to Westhaven.The project will ... unlock the full benefits of the North Shore busway.
