Auckland Mayoral hopeful John Tamihere's plan to beef up the city's Harbour Bridge by giving it 18 lanes has been slammed by Associate Transport Minister Julie Anne Genter.

Tamihere's plan - which he claims would cost $2 billion while his opponent estimated closer to $10b - would include 10 car lanes, and an upper level with four rail tracks, two cycle lanes, and two walkways.

Speaking to media this afternoon, Genter said the idea was "absurd" and that Tamihere had a "very poor understanding of how transport systems work".

Tamihere had previously said his team had looked into the costs and concluded it was "very doable".

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But Auckland Mayor Phil Goff has estimated Tamihere's bridge would cost some $10 billion to build.

He branded the plan "fantasy stuff".

Genter was inclined to agree, and questioned where Tamihere expected to get the money from.

"I think that Mr Tamihere has demonstrated that he has a very poor understanding of how transport systems work and also how Government funding works."

The project would be inefficient and not what Aucklanders want, she said.

"Aucklanders really want a rapid transit system, not heaps more traffic in the city centre."

Genter's comments come after Tamihere and Goff went head-to-head in an NZ Herald debate last night.

Tamihere accused Goff of introducing four of the eight taxes on Aucklanders' rates bills while Goff repeatedly accused Tamihere of coming up with policies he knows he cannot deliver.

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Auckland Mayoral Debate - Phil Goff vs John Tamihere

"Everything John says is factually incorrect. He makes it up as he goes along," he said.

Tamihere proposed turning the existing Harbour Bridge into a two-level superstructure – with 10 lanes for cars on the lower level and four rail tracks and walkways and cycleways on the upper level.

The superstructure replacement for the 60-year-old bridge is the first transport policy announced by Tamihere, who is challenging Phil Goff for the mayoral chains at October's local body elections.