That involves digging "cut and cover" tunnels west from Britomart and then through Albert St to Wyndham St, to clear the way for a major redevelopment of the Downtown shopping centre in the path of the 3.5km rail link to Eden Tce.
But he is proposing a two-year delay, to 2018-19, for the start of the main tunnelling and construction of two underground stations - which would push the completion date to 2023.
Even that is too ambitious for new Transport Minister Simon Bridges, who said last night the Government remained committed to a 2020 start.
"The Government would only consider an earlier start date if it becomes clear Auckland's CBD employment and rail patronage are growing faster than expected. To date, all indications are that this is unlikely to occur."
Mayor Brown was keen to highlight the Government's agreement in principle last year to support a project it previously opposed.
"We have moved them from a position of total opposition to one of commitment for funding half the project from the year 2020," he said.
The co-ordinator of Auckland's new Public Transport Users' Group, Jon Reeves, said Britomart was reaching maximum capacity as a cul-de-sac station able to handle only 20 trains an hour.
"What happens when it hits its maximum - are we going to be sitting around for eight years twiddling our thumbs?"
Labour transport spokesman Phil Twyford said every week without the rail link cost Auckland jobs and growth, and the delay caused by the Government's refusal to commit funds before 2020 would set back the city's prosperity.
But right-leaning Waitakere councillor Linda Cooper said that although she "absolutely" supported the project, Ms Provost was right to call out the mayor on his attempt to start the project without the financial means to follow through on it.
"You can't buy a house if you don't have a guarantee from the bank."
Orakei councillor Cameron Brewer said he was pleased the Audit Office had now stepped in.
"Last month, I said there was more chance of Father Christmas coming to the party next year than the Government but was scoffed at, with the mayor giving councillors absolute assurance that this approach was the right and responsible one. He has now thankfully been pulled into line."
Council infrastructure chairman Mike Lee described the mayor's compromise proposal as "pragmatic" even if it meant more congestion in the meantime.
Councillor Christine Fletcher called the development "a hiccup" rather than a major obstacle.
Additional reporting: Susan Strongman