Transport Minister Phil Twyford has received official advice to do more work on using buses instead of modern trams from downtown Auckland to the airport - a move that could save taxpayers as much as $2.5 billion.

But the minister, who says trams to the airport has a preliminary estimate of $3.7b, has rejected the idea of running buses on a dedicated corridor. Officials have put a cost of $1.2b to use advanced buses to the airport.

Prime Minister Jacinda Ardern and Twyford made an election promise last August to build modern trams, or light rail, from the CBD to the airport and from the CBD to West Auckland. The latest cost estimate for the project is $6b.

Papers obtained by the Herald under the Official Information Act show Twyford was advised by the NZ Transport Agency (NZTA) shortly after coming to office in November to look further at advanced buses to the airport before finalising the preferred mode.

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Officials said new transport technology was evolving quickly and narrowing the difference between the levels of service and capacity between advanced buses and modern trams, citing an example being tested in China.

"These technologies (of the buses) have the potential to deliver light rail performance at lower cost due to less track and fewer overhead infrastructure requirements.

"Before finalising the preferred mode, the proven ability of these systems to deliver similar levels of service at significantly lower cost should be confirmed," NZTA said in a briefing note to Twyford dated November 1.

Transport Minister Phil Twyford. Photo / Jason Oxenham
Transport Minister Phil Twyford. Photo / Jason Oxenham

Last night, Twyford said NZTA and Auckland Transport made a decision under the former National Government that light rail was the most appropriate form of mass transit from the city to the airport.

Since receiving the briefing note on November 1, Twyford said, officials from AT had visited China to look at the bus technology using autonomous vehicles with rubber wheels running on rails.

The officials found for a number of reasons the technology was not suitable for Auckland. It was unproven and not in commercial use outside China, was slower than light rail and battery powered requiring longer stops at the end of the lines, he said.

This month, Twyford and Finance Minister Grant Robertson launched a procurement process for a modern light rail network to "transform Auckland", saying the Government had received an unsolicited approach from the Super fund to design, build and operate it.

Work done by NZTA and AT last year on an advanced bus study for Auckland was picked up by then Transport Minister Simon Bridges to protect a mass transport route to the airport for buses before changing to light rail after 30 years.

The study found advanced buses could provide a credible solution over the next 30 years.

National's Transport spokesman Jami-Lee Ross said for Labour to be proceeding in an ideological focused way with light rail regardless of the evidence and advice is a reckless use of money.

Proceeding with $6b of light rail was coming at a cost of many other transport projects in Auckland, Ross said.