Key Points:

Cycling and walking campaigners are trying to open a new front in their battle for access to the Auckland Harbour Bridge.

They want the Transport Agency to consider slinging a cycle-walkway under the main truss bridge to get around the organisation's concerns about adding "dead-weight" to the traffic clip-on structures.

The agency's board has rejected a recommendation by consultants to widen the outer edge of each clip-on by 1.2m for a cycling path on the western side of the bridge, and a walkway on the other, for up to $43 million.

Its engineers warned that the extra load of such extensions, combined with concrete barriers to protect walkers and cyclists from bridge traffic, could knock 10 years off the economic life of the clip-ons - accelerating the need for $4 billion of harbour tunnels.

But the Getacross Campaign is now suggesting the agency examine in greater depth an option for building a foot and cycle crossing under the main bridge, thereby keeping any extra load off the clip-ons.

Campaign spokesman Bevan Woodward said he has been told informally by Auckland Regional Transport Authority staff that a basic under-slung structure, such as on the Gardiner motorway bridge across the Yarra River in Melbourne, should cost around $10 million.

That compared with "decongestion" benefits estimated by his authority sources of $42 million over the life of the structure, if 1000 cyclists used it each workday instead of driving or catching buses over the bridge.

The Transport Agency is sceptical about Mr Woodward's information, saying he grossly underestimated the cost of adding a cycleway to the clip-ons.

Acting regional manager Tommy Parker said an under-slung option was considered last year in a $200,000-plus study of the feasibility of adding walking and cycling links to the bridge, but was discarded on security grounds.

He said the regional transport authority, a partner in the study, accepted that finding.

Mr Parker acknowledged that a structure supported by the main truss bridge would not impose any extra deadweight on the clip-ons, but said it would be difficult to maintain and safeguard, being out of sight of traffic passing above.

"If you look internationally, they are not easy to control - they are not pleasant places to be. They are always problematic."

He confirmed that he and agency regional director Wayne McDonald refused to discuss the idea with the campaigners at a pre-Christmas workshop, leading to comments that walking and cycling on the bridge was a "dead horse".

"Bevan's trying now desperately to open up different avenues, but we've put it to bed."

Mr Parker insisted the agency was not against walking and cycling elsewhere, saying it had applied to build $17 million of links next to state highways through the region in the three years from next July.

Mr Woodward said he believed antisocial behaviour on a cycleway beneath Mangere Bridge had jaundiced the agency's views, and suggested that a facility over Auckland's main transport lifeline could become an attractive and well-used showpiece.