Treasury is cautioning the Government that its chosen way forward for Auckland light rail could "dilute accountability" and further delay a project that is already years behind schedule.
That is also the view of the Infrastructure Commission, which noted that Transport Minister Michael Wood's attempt at an inclusive approach was not international best practice.
The concerns are revealed in Wood's Cabinet committee paper, which also considers how to find the billions of dollars that will be needed to build it, including possibly raising petrol taxes.
Last week Wood announced the next step forward for light rail - an establishment unit between central and local government to come up with the mode, route, costings and financing options for Cabinet to consider later this year.
The unit will be overseen by a board which includes an independent chair, chief executives of the Ministry of Transport, Auckland Council, Auckland Transport and Waka Kotahi, Treaty of Waitangi partners, a local board representative, and observers from Treasury and the Infrastructure Commission.
But Treasury said the board was too broad, and mixing stakeholders and advisers with those responsible for setting up a new business case was fraught.
"We are concerned ... [it] will dilute accountability and could result in delays due to a lack of clarity around decision-making," Treasury said in the paper.
"It is critical for the future of the project that there is as broad agreement as possible on key choices such as mode and route alignment. However, the Treasury's recommendation, aligned with international best practice, is that this is most effectively achieved through a separate stakeholder and/or expert advisory group."
The advisory group would ensure stakeholders' views were taken into account, Treasury said.
"These views are also held by the NZ Infrastructure Commission."
Act Party transport spokesman Simon Court, formerly a civil and environmental engineer, agreed it was a mistake to include stakeholders on the board.
"Each stakeholder will have separate agenda and, having delivered infrastructure projects in Auckland myself, I know it's just not possible to please everyone all of the time."
Wood said that Auckland Council had also raised concerns about the board's ability to make critical decisions in a timely way.
But he said these were outweighed by the need for "inclusivity, building consensus and social licence", given the current absence of a plan.
"The role of the independent chair will be critical in bringing together the diverse composition of the board," Wood said in the paper.
Woods is hoping for shovels in the ground by the next election for a project that has already been pushed back by years; Jacinda Ardern promised as Labour leader in the 2017 election campaign that the first leg of the project - from the CBD to Mt Roskill - would be completed within four years.
The establishment unit will investigate costs, but Wood said they will inevitably be high.
Funding should be a combined approach "across Crown, NLTF (National Land Transport Fund), farebox, local government tools (to capture local/regional community benefit), private funding sources (to capture commercial/private business benefit) and new value capture mechanisms".
Value capture could see property owners bearing the costs of projects that increase the value of their land.
"Agreements with property owners, public and private sector partnering for development, and targeted rates may all play a role in this funding toolkit," Wood said in the paper.
Seed funding of $1.8 billion is committed from the NLTF, but if more is needed from the NLTF, it would see either other transport projects losing out or an increase in petrol taxes and road user charges.
Similarly, a Crown grant would come at the expense of "wider government priorities", or it would require more debt.
Ardern has said it's too early to consider how much it will cost and where that money will come from, but she has previously ruled out any more regional fuel taxes for as long as she is PM.
Once the establishment unit came up with a plan and business case, Cabinet will make the key decisions on the route, mode and who will deliver the project.
The Herald understands it will be six to eight years before light rail is up and running in Auckland.