Auckland Mayor Phil Goff believes the second Auckland Harbour crossing should have a busway, instead of a rail line, when it is first constructed.

Auckland Mayor Phil Goff would prefer the city's second harbour crossing to be built with a busway instead of a rail line.

Goff said the $4 billion tunnel under Auckland Harbour, planned for about 2030, should be built with a busway to begin with.

"Busways are easily translatable to light railways, so the two are quite compatible. You may sequence it in that order. That's my preference," Goff said.

PHIL WALTER Auckland Mayor Phil Goff would prefer the tunnel to have bus lanes rather than rail.

Goff said he was keen for a rail line to Auckland's North Shore, eventually, but a rail line to the airport was a higher priority.

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The Additional Waitemata Harbour Crossing will be funded by the New Zealand Transport Agency (NZTA) which is responsible for funding all state highway projects throughout the country.

Bevan Read Auckland Mayor Phil Goff said it was his preference for the planned $4 billion tunnel under Auckland Harbour to include a busway to begin, that could be converted into a rail line later.

There has still been no definitive commitment from NZTA to including a rail line to Auckland's North Shore in the additional harbour crossing - only that it will include a "mass transit" component.

Green Party transport spokesperson Julie Anne Genter said it would be "crazy" to build a busway in the AWHC that was just going to be converted to a rail line later.

"Our population is growing really fast and we already need extensions to the rail network. So, I don't know why we would bother building a busway first, which would then need to be disrupted during construction to be transformed into rail," Genter said

SIMON MAUDE/FAIRFAX NZ The eastern side of Auckland Harbour Bridge where the Additional Waitemata Harbour Crossing tunnel is planned to be built.

While Genter admitted a busway would initially be cheaper, she said: "Over the long-term life of the project, it will cost us less and benefit us more to do rail first."

Genter, however, stressed that the mayor's position on rail to Auckland's North Shore had been forced on him by the National Government's lack of transport funding.

"I think that the mayor is in a difficult situation because the National Government is refusing to allow Auckland to raise revenue to invest in long-term infrastructure through a regional fuel tax, and that's what needs to happen," Genter said.

STUFF Green Party transport spokesperson Julie Anne Genter says the lack of funding from the National Government for Auckland transport infrastructure has but Mayor Goff in a "difficult situation".

"It's really not the mayor's fault, it's our National Government who's holding Auckland back from investing in the long-term infrastructure that we need and want."

But despite Goff's confidence that a busway in the Additional Waitemata Harbour Crossing could be easily converted to a light rail line, TransportBlog spokesperson Matt Laurie has his doubts.

"From my point of view, you would either build the crossing with light rail, or you go with buses the whole way," Laurie said.

"I've never heard of a tunnel that you'd convert from being a busway into a light rail in the future. You'd need to design it with the various safety standards in place for whatever method it's going to be.

"One of the advantages of a rail-based system is that it's stuck on rails and it's not going to move off a set margin, whereas in a bus the driver is going to veer around a bit and he needs a bit more space in the road corridor."

A poll from Generation Zero in 2016 found that 64 per cent of Aucklanders wanted a rail and road AWHC. Only 22 per cent of people wanted a road only crossing.

However, the Auckland Transport Alignment Plan, released in September 2016, indicated the AWHC may not be built and open to commuters until 2048 - a blow out in NZTA's initial time estimate of 2030.

Mayor Goff talked to Stuff about transport and other issues in a Facebook Live Chat on February 16.