Options for a new Waitemata Harbour crossing in Auckland have been narrowed to a short list of two routes.
One could be covered either by a 4.5km tunnel or a sweeping bridge.
Only a longer tunnel, of up to 6.8km, would suit the other route, says the team conducting a $1.3 million study for Transit New Zealand, the Auckland Regional Transport Authority and city councils on both sides of the harbour.
The study has narrowed more than 160 crossing options down to three - two going between Esmonde Rd in Takapuna and Wynyard Wharf, crossing the harbour by either a tunnel or a bridge before continuing underground through central Auckland.
One would be only for public transport, whether buses or a rail extension to North Shore City; the other would be shared by cars and trucks.
The third option is a longer crossing, between Esmonde Rd and Grafton Gully to the east of Auckland's commercial hub, but with a public transport branch to Britomart from under the harbour.
That would have to be a tunnel, because of the impracticality of building a bridge high enough for container ships to pass below.
But digging an "immersed tube" tunnel into the harbour bed would also affect port operations, and the study is vague on where it would intersect wharves.
Gone are ideas of a crossing west of the existing harbour bridge, which the study team wants to keep for general traffic, and for pedestrians and cyclists if a tunnel is to be built to the east.
Although the study partners have yet to endorse the short-list for detailed assessments before a final decision in March, the exclusion of options such as a crossing between Waterview and Birkenhead, or between Mechanics Bay and Bayswater, has been welcomed in North Shore City.
"There were so many options that they virtually passed through everybody's place from Birkenhead to Devonport and created so much alarm," said deputy mayor Julia Parfitt.
"This provides some certainty, and seems to be connecting with the motorway and our busway artery."
Study director Richard Hancy said a bridge would be cheaper to build than a tunnel, but detailed costings were not part of the initial phase of an exercise aimed at protecting the best route against future development.
In 2003, Transit produced a $3 billion estimate for a tunnel under Wynyard Wharf.