The eastern side of Auckland Harbour Bridge where the Additional Waitemata Harbour Crossing tunnel is planned to be built.

The $4 billion tunnel to be built under Auckland Harbour sometime between 2030 and 2050 will not improve traffic congestion if it is a road-only crossing, a report says.

The Green Party has highlighted the New Zealand Transport Agency's 2010 report in the wake of the recent Auckland Transport Alignment Project plan.

The plan provided the strongest indication yet that NZTA and the Government will fund a rail link to Auckland's North Shore.

Dave Moore Bird's eye view of peak hour traffic along the Esmonde Rd entrance-way to the Auckland motorway leading to the Auckland Harbour Bridge.

NZTA projects in the report that in 2026, there would be little to no improvement on the North Shore's Esmonde Rd and Onewa Rd - both notorious bottlenecks leading onto the motorway - if a road-only harbour crossing were built.

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It also predicted there would be little to no improvement on Fanshawe St and Grafton Gully in the CBD.

Supplied Green Party transport spokesperson Julie Anne Genter.

The NZTA report - "Additional Waitemata Harbour Crossing (AWHC) Network Plan: Local Roads" - states "models are indicating that the AWHC option will result in similar queuing in the city centre".

It also states the queues along Esmonde Rd and Onewa Rd "will be similar without and with the AWHC option".

The alternative option is a combined road-rail tunnel under the Waitemata Harbour.

ZIZI SPARKS/FAIRFAX NZ Peak-hour traffic along Auckland's Northern motorway.

A NZTA statement says their 2026 traffic projections, made in 2010, are now out of date:

"A road-only AWHC is not being considered and the reports being referenced are from a previous study from 2010 which included modelling information for 2026. Given the long term nature of the project, the traffic and transport modelling information will need a longer term view."

Green Party transport spokeswoman Julie Anne Genter says the Government's reluctance to commit a rail component to the tunnel is due to a historically narrow focus on state highways.

Joel Ford Minister of Transport Simon Bridges.

"I think the issue here is NZTA have a particular responsibility for the state highway network and they tend to ignore the rest of the transport network in and around the city," she says.

"They are not willing to investigate a rail-only option even though arguably that would be the most effective way of helping people get around the city, as well as improving the flow on the state highway, because it gets the commuters off the state highway."

In a statement, Transport Minister Simon Bridges would not be more specific than saying either a bus or rail link will be included in the third harbour crossing.

Jason Oxenham/FAIRFAX MEDIA Traffic congestion on Auckland Harbour Bridge is projected to improve with the Additional Waitemata Harbour Crossing but there is less certainty about surrounding suburban streets.

"While the Government hasn't yet decided when it will be required, and precisely what form it will take, in a rapidly growing region it's essential that we protect and keep future transport options open," he says.

"NZTA is working together with Auckland Transport to ensure any designation for an additional Waitemata Harbour crossing is future-proofed and enables a rapid transit, public transport crossing.

"Both organisations are currently investigating which modes of rapid transit, whether it's bus or rail, will best service the growing needs of the city."