Auckland's mayor Len Brown wants to get his $2.8 billion rail project underway four years ahead of schedule and has told the Government the city can bankroll the early stages.
Prime Minister John Key last year said the Government was committed to paying half the cost of the project, but this has yet to be confirmed.
The proposal has the project starting in 2020 but Brown has written to Key saying he wants it started by 2016.
In the letter Brown has offered $250 million of city money to get the project started earlier, covering the first four years' work.
Brown told Radio New Zealand he wants the project, which includes an underground railway loop through the city, underway earlier because of growing private interest in it.
"Just like any other international city where you build this type of infrastructure, you get massive private sector investment and big big gains in the commercial world," he said.
"We are really only just warming up."
He said it was a significant and game-changing project and decisions were needed over the next few months.
"The commercial sector are making decisions and in many ways they are now driving this project," he said.
Brown said that waiting until 2020 would worsen disruption when the tunnel was being built.
Auckland Council has spent more than $100 million on property purchases and other route-preparation work.
The New Zealand Herald said Brown's letter to Key said the city's offer to start earlier was based on a government commitment to pay for half of the project.
"Effectively, council would be underwriting the Government's contribution until 2020 (or an agreed earlier date)," Brown wrote.
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