As the business case for Auckland's proposed $2.4 billion rail tunnel went under a Treasury blowtorch, Super City leaders turned to international consultants to shore up their cause.

The result is a yawning gap between predictions of what the mega-project would mean for Auckland - whether it would help to provide the city with a pulsating hub or whether it would be little more than a colossal money-swallowing hole.

Transport Minister Steven Joyce, despite saying the case for building the 3.5-kilometre link has not yet been made, is not ruling out its ultimate construction.

"I think longer-term it is the most likely next rail project for Auckland given the resilience benefits and the strategic arguments sort of make sense," he told the Herald.

But he says KiwiRail has no money for protecting the route in the meantime, and that Aucklanders rather than the Government must be prepared to pay if they want the project kept alive.

He is, meanwhile, recommending a more thorough study of the central city's transport needs, including whether more bus projects and park-and-ride stations could provide better uses of existing networks.

Mayor Len Brown is being diplomatic in accepting the challenge, refraining from criticising the Government while insisting it will eventually be persuaded by the strength of arguments for the tunnel - supported by nine teams of consultants, both local and international.

But others are scathing of Mr Joyce's stance, accusing him of subjecting the tunnel to a degree of rigour absent from his own preferred projects - notably the broadband rollout and the $1.65 billion Puhoi to Wellsford highway.

Auckland Council transport committee chairman Mike Lee said the mayor had an enormous mandate from the public to lead a project which was being "smothered and crushed by a man who has no political mandate whatsoever, who is in fact a list MP".

He accused Mr Joyce of making it clear from the start that "his minions ... would go after that business case like wild dogs, and so they did".

"The fact is that the Government, after a very long process, has declined funding for the most important transport project in Auckland's history and this is the third time a National government has done it."

He was referring to decisions to block previous versions of the project, in the 1950s and 1970s.

Although Mr Joyce says scrutiny of the project by the Treasury and Ministry of Transport was a robust exercise, peer-reviewed by a leading British consultancy, Labour transport spokesman Shane Jones accuses him of an "ideological jack-up".

"This is a selective application of criteria, putting Mayor Brown's figures through the grinder while he has offered a blank cheque to Telecom on broadband without any rigorous cost-benefit analysis, and been fast and loose with his figures on the Puhoi extension."

Heart of the City business association chief executive Alex Swney feared the tunnel project was suffering from a similar "myopic mind-set" that limited the harbour bridge to four traffic lanes when it opened in 1959.

That was because Wellington-based decision-makers could not foresee the expansion the bridge would offer.