An independent investigation has been ordered into the SkyPath walk and cycleway across the Auckland Harbour Bridge.

Photo: Copyright © 2011 - 2014 Generation Zero Incorporated.

It comes amid an escalating row between the Transport Agency (NZTA) and the SkyPath Trust which launched the project a decade ago.

The trust is threatening to call for a protest march across the bridge.

Early yesterday, the NZTA chair Michael Stiassny wrote to the trust saying it had not allowed its design to be assessed and this has "slowed the process considerably".

"The trust has not yet allowed the Transport Agency to access or assess the necessary documentation," he said.

"But be very clear, I cannot, and will not, allow public money to be used to pay for information that we have not assessed and may be of no value," he wrote.

The trust rejected that as "laughable", saying the agency had been partners with it and had access to assess its design for years.

Last night however, Mr Stiassny issued a statement saying he was being given contradictory information.

"I wrote to the SkyPath Trust this week to propose a way forward and to provide my assurance as NZTA Board Chair that we will deal with the Trust in good faith, including paying for any intellectual property that we use," he said in the statement.

Photo: RNZ / Jo Moir

"I have received correspondence from the SkyPath Trust today which includes statements which contradict my understanding of the NZTA's previous dealings with the Trust. I am deeply concerned with this situation and I have ordered an independent investigation to establish the facts once and for all."

Mr Stiassny declined to be interviewed.

The $67m SkyPath has been on the drawing board since marchers blocked the bridge in 2009 in protest at not being allowed to walk on it.

Yesterday the trust's chair Christine Rose was not ruling out a repeat of that.

Photo: Supplied / Brian Moorhead

"Make no mistake, the demand and public support for this project have not gone away, and in fact, we know there are people who would repeat what we achieved previously when 5000 people took to the bridge in defiance of NZTA," she told RNZ.

"There's a real appetite to do that again."

This came after Mr Stiassny, in his letter to the trust, blamed it for the standoff.

Ms Rose scoffed at the suggestion the agency did not already know its designs in depth.

"That's not credible, it's not honest, I don't know if it's willing disingenuousness, or dysfunctional, or political obfuscation.

"But SkyPath has been very transparent. He should possibly consult some of the work that's been done in partnership with NZTA."

Documents support her contention the agency has kept itself well abreast of the project for years: In 2011, architects wrote to the agency about it engaging them on a two-month study to determine the SkyPath Trust project's overall viability; in 2013, an agency memo said the trust's concept had been tested by engineers and was feasible.

Mr Stiassny in his letter yesterday gave an assurance the agency would pay the trust for its design work - if they used it.

"If we use the Trust's full proposal we will pay the full amount of $1.6 million, if we use some of the trust's proposal we will scale the payment appropriately."

Christine Rose said she believes it is reasonable for the trust to be cautious disclosing its design documents - but added that the project would not have got this far without the design already being put through its paces in the resource consent and Environment Court hearings.

The government on Monday called for the trust and agency to begin talks to reach a solution.

It remains unclear what the new invesigation announced by Mr Stiassny will cover.