Not even a $2.86 billion rail tunnel will stop morning traffic in downtown Auckland from grinding down to half its current speed within a decade, according to a major study recommending the project.

But without it, there will be inadequate road capacity to meet demand after 2021, and relying solely on more buses to improve public transport will hasten gridlock, say consultants hired by Auckland Transport to answer Government questions about a 3.5km rail tunnel from Britomart to Mt Eden.

Even so, the Sinclair Knight Merz consultants are recommending extra buses and underground trains to ensure enough people can be brought into central Auckland and its fringes to fill extra jobs predicted for the country's commercial capital.

That option - costing about $3.3 billion after inflation - is expected to slow traffic entering the city between 7am and 9am on weekday mornings to about 6km/h by 2021, compared with 16km/h now.


Although a rail tunnel for extra public transport would offer motorists a slightly higher speed of 8km/h, Auckland Transport chief executive David Warburton says an integrated solution involving more buses would be the only way of bringing enough people into the city centre.

The consultants predict the number of people working in central Auckland and its fringes will double to more than 200,000 by 2041, although the Government is questioning that.

Their study was hailed by Auckland Mayor Len Brown as providing a strong basis for entering funding negotiations for the rail project.

Transport Minister Gerry Brownlee said the study was a useful addition to the debate on long-term transport management in central Auckland but fell "some way short of convincing the Government it should provide financial support to any fast-tracking of the proposed city rail link".

He believed an assumption of central city employment growth of 46 per cent in the next 10 years, compared with an increase of just 18 per cent in the past decade, was too ambitious.

And he said the report underplayed the role of highway improvements such as completing the western ring route in 2017, which would draw many thousands of traffic movements away from the city centre.

Dr Warburton said although more work was needed on the business case, the most important consideration for now was its favourable comparison with bus-heavy options.

Mr Brown said he appreciated Government funds were constrained by the recession and the Christchurch rebuild, but delaying the project would limit development throughout Auckland, not just in the city centre.


Labour transport spokesman Phil Twyford believed the Government was "hellbent on stopping Aucklanders fixing the gridlock" and his Green Party counterpart, Julie Anne Genter, said the Government could not argue that the cost of the project was too high when it planned to "waste billions of dollars" on low-value roading.