Road tolls are to be considered for a new $3 billion-plus crossing of the Waitemata Harbour and the existing harbour bridge.

The Transport Agency is asking consultants to consider tolls and public-private partnerships as possible funding sources for a new crossing.

If tolls were charged on it, motorists may also be charged to use the existing bridge to avoid overloading it at the expense of the next crossing, which is to become part of State Highway One.

Questions over whether the crossing should be road and rail tunnels or a new bridge over the same 3km route between the Tank Farm and Takapuna are also included in documents the agency issued yesterday for bids for three sets of investigations, to be completed in October.

Agency acting regional director Tommy Parker said that because the Government had included the crossing in its first 20-year national infrastructure plan, his organisation was trying to "get a move on things".

"We've got a mandate from our board to progress with this and we're going to keep it moving," he said.

Although a 2008 study for the former Transit NZ and Auckland councils evaluated more than 150 route options before recommending a set of four tunnels for $3.7 billion to $4.1 billion, promoters of a new bridge commemorating the centenary in 2015 of Anzac Day are thrilled their proposal is to be considered.

"This is good news for our bridge initiative as this crossing is no longer defaulted to the tunnel option," said spokesman Richard Simpson, a former Auckland City transport committee chairman.

"The new bridge we are proposing offers far greater resilience than the old bridge that is facing failure and escalating maintenance costs with the increased weight of trucks and traffic demands.

"A bridge is also considerably more resilient than a tunnel alternative."

But North Shore Mayor Andrew Williams said it took several years of co-operation between various councils and agencies to reach consensus support for tunnels and another bridge would have an unacceptable impact on communities on both sides of the harbour.

He said it was hypocritical for Transport Minister Steven Joyce to relitigate that process while claiming it would take too long to allow Auckland councils to agree on appointees to the inaugural board of the region's new transport super-agency.

The minister, who intends appointing the board with Local Government Minister Rodney Hide, resurrected the debate in December by saying he was not knocking tunnels but the size of the required investment meant the Government needed to be convinced about the best approach.

He said then he wanted a new crossing built within 15 years.

Mr Parker defended the $1.3 million study published in 2008 as robust, but said his organisation needed to consider economic realities in calling for cost-benefit evaluations of bridge and tunnels options.

One assumption of the previous study had been that the new crossing should work around plans for a multimillion-dollar redevelopment of the Tank Farm.

Although he estimated that a bridge would cost about $600 million less than tunnels, he said construction timing would be less flexible and the agency would need to consider the loss of development potential an overhead structure would inflict on the Tank Farm.

The Anzac Bridge group says $1 billion could be raised for the project by closing the existing bridge and selling off motorway land around St Mary's Bay.

But Mr Parker said although that idea would be looked at, the agency's strong preference was to retain the existing bridge to run parallel with the new harbour crossing for maximum transport network resilience.

Mr Williams said his council always expected some form of tolling would be needed and recognised the difficulty of keeping the existing bridge as a free route in that event.

He acknowledged North Shore residents had already paid heavily for the bridge through paying tolls for 25 years until 1984, but said they might feel happier about new charges if these were also levied on other parts of Auckland.