Auckland's Super City leaders are forging ahead with securing a route for a $2.4 billion central railway tunnel despite the Beehive throwing cold water on the project.

Mayor Len Brown yesterday confirmed a $2 million initial commitment of ratepayer cash to project investigations. He will put a longer-term budget to his councillors before Christmas.

That will be for buying property along the tunnel's 3.5km route from the western end of Britomart to Mt Eden, which Mr Brown hopes will form an inner-city rail loop in five to seven years.

His determination contrasted with Transport Minister Steven Joyce's concerns that an economic case for the rail link had yet to be made.

Mr Joyce said a Government review had found that a glowing business case on the link produced last year for KiwiRail and the former Auckland Regional Transport Authority was "under-cooked", and far more work was needed to assess the central city's transport needs.

He said any plan should include consideration of other transport modes, notably buses.

The Government review found that "transformational" economic benefits of $3.3 billion claimed by the business case were significantly overstated and the likely return on each dollar invested would be only 40c.

That compares with a return of $1.10c to $2.30c predicted by a report which the Auckland Council and its transport subsidiary commissioned once they saw where the Government review was headed.

Even so, Mr Joyce agreed it made strategic sense for the Auckland Council to move to protect the route in the meantime, if that was what it wanted to do.

"It's up to them, because it's their money," he said.

He wouldn't rule out future Government funding for the project - if a robust case could be made.

"We're not saying no - we're just saying there's a long way to go."

Mr Brown expects to win the Government over.

"I am totally confident that in the months and years ahead we will bring the Government on board with the strength of our argument, the clarity of our case, and the determination of this council ... and our community that this is a project critical to Auckland."

One clear point of difference between the reports has to do with the capacity of Britomart if it is left as a dead-end terminal rather than being made part of a rail loop.

The consultants believe it has only a few years of capacity left, but Government officials say constraints on the western railway line will not be exceeded until 2026.

The latest Auckland report says the new tunnel would enable at least 20,000 more people to travel to the central city on trains during morning peak periods by 2041 compared with the Government's estimate of 6000.

PricewaterhouseCoopers director Chris Money, a former senior Ministry of Transport official, said that was because the Government report had extrapolated from past trends to predict Auckland's growth.

But the report he helped to prepare was focused on the region's new spatial plan of greater urban intensification.

Labour's transport spokesman, Shane Jones, said the gulf between the reports showed Mr Joyce's rejection of Auckland's case was not credible.

"Aucklanders are telling me his policy position is equivalent to a hate crime against rail."

But National's MP for Auckland Central, Nikki Kaye, believed the project would ultimately proceed.