A Treasury report puts the cost of the preferred option for a second Waitematā Harbour crossing at between $23 and $27 billion.

The Transport Agency and Auckland authorities had been looking at, and bickering over, what to do about relieving pressure on the Harbour Bridge for years.

A business case was completed several months ago, and it's current recommended option included both a tunnel and second bridge, according to a newly released Treasury investment report.

It also encompassed upgrades to the Northern Busway, Harbour Bridge and State Highway One.

"The total cost of Stage 1 capex [capital expense] is $22.9- 27.2 billion," it said.

The government rejected a plan hatched under it's predecessor costed at over $50 billion.

"To ensure that the project achieves value for money, I have directed NZTA to refine the scope of the crossing and develop a more detailed understanding of the costs, risks, trade-offs, and assumptions for the tunnel and an equivalent bridge option, prior to seeking a decision from ministers on which option to progress," Transport Minister Simeon Brown told RNZ in a statement on Wednesday.

Auckland mayor Wayne Brown said a second bridge will be "hugely cheaper, massively cheaper." Photo: RNZ / Marika Khabazi

Auckland's mayor Wayne Brown recently urged the government to abandon the tunnel option completely. A second bridge was "hugely cheaper, massively cheaper", he said.

The new investigations would take 12-18 months, NZTA Waka Kotahi said, which would include sounding out potential partners on what they prefer, and taking "market sounding to understand market appetite for options".

It had yet to do a geotechnical study either - Brown said this might start as soon as the next few weeks.

The agenda must also update consenting pathways and protect the route, and look into funding and contract options, Brown said.

All major highways must now consider charging tolls. Public-private partnerships - PPPs - were also much more on the table.

A detailed business case may begin next year, with the construction itself from 2029-40, the Treasury report said.

So far, at least $36m had been spent on the preliminary business case, three quarters of that on consultants.