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Transit NZ is considering laying the Auckland end of a $3 billion harbour tunnel below the proposed Tank Farm redevelopment.
That would allow two mega-projects to be dovetailed through the 35ha prime waterfront precinct, which is earmarked for $2 billion-plus of above-ground developments.
A proposal to double the width of Daldy St to 40m and turn it into an 800m-long park through the spine of the Tank Farm would leave Transit enough underground space to extend the tunnel from the harbour to a possible motorway connection near Cook St, on Spaghetti Junction.
The tunnel could emerge through a portal next to the southern end of the $320 million motorway tunnel which Transit intends digging through Victoria Park by 2012 for harbour bridge traffic.
Transit also suggested yesterday that an underground bus or even light rail station could be built beneath the Tank Farm during development work expected to last 20 to 25 years.
Regional manager Peter Spies said the Daldy St corridor would be wide enough for a cut-and-cover tunnel to be dug with four lanes - two for public transport and two for general traffic - although Transit would also look at other possibilities in a joint study with the Auckland Regional Council and other parties.
He compared it to the Britomart tunnel and allowances made in designing that project for ambitious building projects above ground, although the reverse order would apply to the waterfront development, which is expected to start next year.
A harbour tunnel would not be built until at least 2020.
"For the development of Wynyard Quarter it is probably likely you would have the reverse," Mr Spies said. "So as they design the buildings and their basements, simplistically one side of the basement could be for carparking and the other side of the basement wall will be the wall for a tunnel."
He said the tunnel would be designed as immersed tubes dug into the bed of the harbour from Northcote, before climbing towards Wynyard Wharf, through which it would become a cut-and-cover project.
Buildings would be kept clear of the corridor under proposed district and coastal plan changes lodged yesterday by the regional council and Auckland City, and the Daldy St park would be redeveloped above it.
Mr Spies said sites for a possible underground station, whether for buses or for light rail, were also being considered in discussions between Transit and other parties to the study such as the city council and the Auckland Regional Transport Authority.
A station could be built near a possible bus entry and exit point for the tunnel at Fanshawe St, with lifts and stairs for passengers to get to and from Wynyard Quarter, or close to an entertainment zone to be developed along Jellicoe St at the upper end of the Tank Farm.
That second location could future-proof the station for the possible eventual development of a light-rail system, for which a branch tunnel could then be dug under Viaduct Harbour to Britomart.
Mr Spies said Transit had been close to applying for a designation to safeguard the route through the Tank Farm before agreeing to working with the regional council, the transport authority and Auckland and North Shore cities on a joint study of that and other options between now and March.
He confirmed that the possibility of a tunnel between Mechanics Bay and Bayswater was one of many options likely to be looked at, although opposition from North Shore would be taken into account.
Regional council chairman Mike Lee is keen on a harbour tunnel to Stanley St on the eastern side of Auckland's central business district, although starting at Northcote.