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Officials scouting new routes across Auckland's Waitemata Harbour are recommending separate tunnels for general traffic and for buses or trains.
At least two - and possibly as many as four - tunnels between North Shore and Auckland cities are under consideration on a recommendation that has ruled out building another bridge.
It is understood twin two-lane tunnels for general traffic and two narrower tubes for public transport may run from Esmonde Rd on the Northern Motorway to beneath the eastern side of the Westhaven Marina.
That would keep them away from much of Wynyard Pt, which Auckland City and the Auckland Regional Council want to redevelop over the next 25 years for more than $2 billion.
From Westhaven, a bus or rail tunnel or tunnels would veer east - under Wynyard Quarter - to Britomart station, leaving southbound general traffic to keep heading under Victoria Park before emerging into the central motorway junction.
Although the public transport crossing may initially be used as an extension of the Northern Busway, it is to be "future-proofed" for at least light-rail cars, for which the $300 million busway has been designed.
Some leaders, including North Shore Mayor Andrew Williams and regional council chairman Mike Lee, want the crossing designed to carry larger numbers of commuters on full-sized electric trains eventually running as far as the airport.
But that would require lower gradients and therefore longer approaches than tunnels carrying light-rail or buses, as well as the reconstruction of parts of the busway.
Transit NZ yesterday confirmed some basic details obtained by the Herald of the officials' recommendation, which was reported to a steering group of local body politicians on Friday, but said it could not disclose other information before council committees could be briefed next month.
The recommendation followed a $1.3 million study overseen by an "implementation executive" of Transit officials and those of the two cities as well as the regional council and its transport authority subsidiary.
Its mission was to pinpoint optimal routes for protection in land designations before Auckland starts hearings in June for district plan changes for the Wynyard project.
Although Transit has not yet acknowledged a need for a new crossing to reinforce the harbour bridge until about 2021, it initially intended seeking a designation through the middle of Wynyard Pt before backing off and agreeing to work with the other parties on various alternatives.
A study team narrowed 160 crossing options to a shortlist of three main prospects, two of which involved building either bridges or tunnels between Esmonde Rd and Wynyard Pt.
The officials ruled out a third option, over a longer route to Grafton Gully to divert general traffic from central Auckland.
But although North Shore City favours a shorter route, some Auckland City and regional council members are likely to push for the Grafton option to be left open, even though it could prove far more expensive than Transit's initial estimate of $3 billion for the next harbour crossing.
Transit regional manager Peter Spies said another bridge was ruled out primarily on urban design grounds, given the high visual impact it would have on the harbour in close proximity to the existing bridge.
Confirming the possibility of up to four tunnels, Mr Spies said it was becoming cheaper internationally to build several relatively small tubes than a single large-diameter structure.
He would not confirm other details, including a suggestion from a source at last week's meeting that the road tunnels may become part of State Highway 1, leaving the existing harbour bridge to carrying short-distance traffic between North Shore and central Auckland.
North Shore MP Wayne Mapp supported the most direct route, but saw what was being proposed as too "gold-plated" and believed a single general-traffic tunnel with one lane in each direction and a two-lane bus tunnel would suffice.
