Walking and cycling campaigners are seething over what they say is a vague report brushing off plans for a tolled pathway across Auckland Harbour Bridge.

Engineering consultancy Beca has in a half-page memo dismissed as "infeasible" a plan for a 4m-wide path under the bridge's southbound clip-on structure.

That follows what the Transport Agency acknowledges was only a limited assessment sought from Beca of 79 pages of plans and calculations prepared by another consultancy for the campaigners, who are backed by Orewa-based coastal developers Hopper Developments. Bevan Woodward, spokesman for the Getacross Campaign, is accusing the agency and Beca of bending over backwards to frustrate the hopes of thousands of Aucklanders for a non-motorised link over the Waitemata Harbour.

"I feel they are not bringing a can-do attitude to this and are looking for reasons to say no," he said.

The campaigners were seeking more technical details before invoking an agreement by which the agency was committed to employing a third-party expert to provide an independent opinion.

Mr Woodward has campaigned for 12 years for a non-motorised crossing. He said the latest proposal carefully considered technical constraints outlined by the agency.

It reduced the width of the proposed pathway to provide an extra margin for safe loadings, even in the extremely unlikely event that the southbound clip-on became jammed with heavy trucks after an accident involving an illegally overloaded vehicle.

He said barrier arms would also be used to keep loads within the margin by limiting numbers on the pathway.

A similar scenario of an end-to-end jam of fully-laden trucks on the bridge's northbound clip-on prompted the Transport Agency to add 920 tonnes of steel to both clip-ons for $86 million after engineers warned of a risk of catastrophic failure.

But a Beca analysis in 2009 indicated there was spare loading capacity in the southbound clip-on, prompting the campaigners to shift their attention to that structure.

Beca's critique of a report by Airey Consultants for the pathway project says that although there is insufficient data to reliably estimate future traffic growth on the southbound clip-on, it appears higher than on the northbound structure.

It says cross-girders available to support the pathway - even after the strengthening project - do not have enough capacity to carry the extra load "and this makes the proposal infeasible".

Transport Agency highways manager Tommy Parker said the organisation spent considerable time and money seeking a way to add cycling and walking paths before receiving the latest proposals.

He said Beca was not asked to make a detailed assessment of the Airey report "because I didn't want them to spend thousands of dollars on it".