Revelations Cabinet went against Treasury advice by agreeing to endorse the Let's Get Wellington Moving transport package has ruffled the feathers of some city councillors who are now being asked to agree to the plan.
Transport Minister Phil Twyford announced on Thursday the $6.4 billion package, with mass transit being the big winner.
But a Cabinet paper shows the Treasury warned against the timing.
"Making an announcement at this stage carries significant risks, as it will raise public expectations of future investment before the costs and benefits of the package are fully understood.
"The Minister's proposal also relies on several assumptions that have not been thoroughly tested, including exploring long-term financing to fund rapid transit."
LGWM will now go before both Greater Wellington Regional Council and Wellington City Council for endorsement.
City councillor Diane Calvert said the revelations about Treasury's advice were disconcerting and she wanted more information.
"We need to understand why they said that but also why the various parties, the Government, the mayor and the chair of GWRC chose to ignore that."
City councillor Andy Foster said there were clearly a lot of risks in the package and there were currently more questions than answers.
"It would really have been good to have some of the information, which you'd logically expect to be part of the announcement, actually being part of the announcement. We don't have that yet."
Foster said he expected to have more information before making a decision on any endorsement, which would likely include "a long list of subject-tos".
"The question is how much information is available."
LGWM is light on detail.
The plan will be delivered over 20 years, but there are no real solid timelines.
Mass transit will be a priority but whether that'll be light rail or trackless trams remains to be seen.
Exactly what grade separation at the Basin Reserve will look like is subject to detailed investigation.
City councillor and LGWM governance group member Chris Calvi-Freeman said he wasn't overly concerned about the Treasury's advice.
He said there was significantly more work underpinning the project which was yet to be released, but would be as the months go on.
"I don't believe we should have any hesitation as a council in endorsing the whole package in terms of our support for the package in principle.
"We will then only vote for each particular aspect of it to go ahead when we're assured that there is a sound business and when we're assured that the funding is lined up."
Mayor Justin Lester said he was made aware Treasury had advised caution prior to Thursday's announcement.
"Treasury makes a lot of recommendations that the Government will either accept or decide not to accept but ultimately Government is elected to govern and be the decision makers of the day."
GWRC chairman Chris Laidlaw said he was not aware of the advice before Thursday's announcement but he was not surprised by it.
"Treasury has one of the most difficult jobs in the country in balancing the books and managing investment and they take a cautious approach. However, sometimes it is necessary to take a risk to achieve a bold vision for the future."
Twyford said Cabinet decided to endorse the indicative package because Wellingtonians had waited decades for this kind of transport investment.
"Officials have done a lot of work crunching the numbers and looking through the cost implications, including on long-term financing for rapid transit. I will report back to Cabinet with a long-term financing proposal in due course."
Regional councillors and city councillors will debate the package next month.