A report for Auckland Council has cast doubt on the targets set by the Government to make an early start on the $2.86 billion city rail link.
The report by consultants PWC says it is important to set targets that are realistic and appropriate for the Government and Auckland Council make an early start on the project.
Last year, Prime Minister John Key said the Government would bring forward the project from 2030 to 2020, or earlier if the set targets were met.
These were that rail patronage was on track to hit 20 million trips well before 2020 and if central city employment increased by 25 per cent.
The PWC report said the rail patronage target was technically achievable, but was not likely to show up until after full rail electrification in 2015 and 2016.
The Government's employment target is "practically unachievable" because of the constraints on spare office space and new office buildings coming on stream prior to 2017 when the targets would be reviewed to make an early start on the rail link.
The report recommended new rail targets in line with current growth of 800,000 new rail trips per year and post rail electrification of 1.5 million new rail trips.
New employment growth targets should be based on declining vacancy rates of existing office space and sufficient new office space to meet forecast employment growth by 2021.