The Government's confirmed it will pay half the cost of Auckland's City Rail Link (CRL), but mayoral hopeful Phil Goff is questioning why the cost estimate has ballooned out to beyond $3 billion.
Associate Finance Minister Steven Joyce and Transport Minister Simon Bridges on Wednesday said the Government and Auckland Council had signed an agreement committing to the deal.
But they've put the cost estimate for the 3.4km underground rail link at between $2.8 billion to $3.4 billion.
That higher figure has Mr Goff looking for answers, saying the original price was $2.5 billion.
"New Zealand taxpayers and Auckland ratepayers are owed an explanation for why local and central agencies may have underestimated the costs. We also need answers around the reliability of their new estimates and whether these can be depended on."
It's estimated the CRL will cut congestion on Auckland's clogged roads by doubling rail capacity and improving travel times.
But Mr Goff fears if the cost goes up further, it'll put more pressure on ratepayers.
"We need assurances that the project will be delivered on time and within budget."
The Government says it'll make the money available progressively once a sponsors' agreement is in place.
"This agreement is an important milestone in the project and it underlines the Government's commitment to keeping Auckland moving," Mr Joyce says.
Meanwhile Auckland Mayor Len Brown is calling the announcement an unprecedented and historic milestone.
"Today I am very incredibly proud to stand with the Minister of Transport and together ensure this project can now proceed and be funded for the future benefit of all Aucklanders."
The agreement between the Government and the council cements broader funding, governance and risk management arrangements and also outlines an independent 'special purpose entity' for the project which will work with Auckland Transport, KiwiRail and others.
The money could be made available as early as 2017.
Once finished, the CRL be one of New Zealand's largest-ever transport projects, and will run from Britomart in Auckland's CBD and connect to the Mt Eden station.
It'll also include stations at Aotea Square and Karangahape Rd.