The Government has agreed to fund about half the cost of the project, but will not make a financial commitment until 2020, unless the council meets rail patronage and downtown employment targets.
Mayor Len Brown is planning to begin work on the 3.5km underground tunnel in 2016 and complete it by 2021.
Officials are confident of starting "enabling works" in 2016 on a short section of the rail route to go with the redevelopment of the Downtown Shopping Centre.
These works include a $270 million cut-and-cover section of the link from Britomart and up Albert St to the intersection with Wyndham St.
But when it comes to the main contract, starting at Mt Eden station and boring underground towards the city, the officials said work could not begin without Government funding.
Last night, new Transport Minister Simon Bridges said that last year, the Government accelerated the start date of the rail link from 2030 to 2020, and it still believed that was a sensible position.
It did not look like the Government's targets for an earlier start would be met, he said.
Watch: City Rail Link, connecting Aucklanders
Mr Bridges acknowledged the council was grappling with paying for infrastructure and transport projects were essential for the region's thriving economy and growing population, but it made no sense to build the rail link before it was "economically sound and sensible to do so".
Mr Brown said the start time was a matter for negotiation and he remained confident of convincing the government an early start was necessary to keep Auckland moving.
"It has always been the case that council will only proceed with the main works of the link once full funding is in place," the mayor said.
Clarification around Government funding and timetabling will satisfy some councillors who oppose or are having second thoughts about an early start on the project.
A Herald survey of councillors found nine of the 20 wanted to defer the main contract until 2020 and nine plus Mr Brown wanted to make an early start. Wayne Walker has reservations about the rail link and Sir John Walker could not be contacted.
Two schedules
Len Brown's timeline
Enabling works 2016-2018, main works 2017-2021
Government timeline
Enabling works 2016-2018, main works 2020-2025