Auckland mayoral candidate John Tamihere is proposing to turn 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.
Tamihere has not costed the Auckland Harbour Bridge proposal, which is part of the motorway network managed and funded by the New Zealand Transport Agency.
Goff quickly savaged the plan - saying it would cost more than $10 billion to build and would "bankrupt the city".
Tamihere is also promising to put the "on again, off again" Penlink highway linking the Whangaparaoa Peninsula with State Highway 1 on "steroids to get Auckland moving".
A new harbour crossing and the much needed Penlink Highway will be priority projects under a John Tamihere mayoralty, he said.
"A new 10-vehicle-lane harbour crossing, which will carry cars, rail, pedestrians and cyclists can be completed within six years on the existing bridge piers, replacing the existing bridge," Tamihere said.
"There will be minimal disruption to traffic. My team have looked at overseas structures and costs and it is very doable."
Tamihere said full or partial superstructure replacements have been carried out on a number of bridges in the United States, including the Milton-Madison bridge, costing US$104 million ($161m), over the Ohio River. It was built in 1929.
But Goff called the "18-lane bridge" fantasy stuff and fundamentally dishonest to promise. It would cost more than $10 billion to build, he said.
"Either he intends to bankrupt the city, burden ratepayers with massive extra rates or he is making it up because he know he is never going to deliver it.
"Widening the motorway at either end to match the 18 lanes would see massive demolition of buildings and destruction of homes and neighbourhoods," Goff said.
"This will cost further billions of dollars that Auckland doesn't have and the government won't pay for."
Tamihere said the Penlink Highway will bring much-needed relief to North Shore motorists.
"I will personally guarantee and commit to the Penlink Highway because it is the right thing to do," he said.
"Ratepayers and motorists on the North Shore need certainty, not hollow promises and backside covering.
"The 7km Penlink road, consented to run between the Northern Motorway and the heart of the Whangaparaoa Peninsula, will reduce traffic flow by an estimated 50,000 vehicles per day through the Silverdale Business area."