The Auckland Harbour Bridge wobbles when enough people walk on it, to the extent it could result in "serious crushing injuries".
Despite this, the Transport Agency this year organised mass celebratory walks across it - cancelled in March due to weather disruption.
Documents show the agency was told years ago how to fix the wobbles but has not done it.
A crowd of walkers of more than 250 per bridge span sets up a vibration that leads to slight swaying, enough to open and close a gap in the deck 58mm wide, documents released under the Official Information Act to Bike Auckland say.
"Structural failure is not anticipated," a memo a year ago to Waka Kotahi said, but there could be localised damage.
The 1975 hīkoi land marchers led by Dame Whina Cooper first experienced the wobbles, and another land march in 2004, too - but it was little reported on.
It was only behind closed doors last year that the alarm went up after an anti-mandate protest march in February.
A short smartphone video shows people walking where the clip-on lane joins the other lanes on the west side, exclaiming in surprise as the gap opens and closes beneath their feet.
"It's terrible," says one man.
"Is it shaking over there?" another calls out to a girl, who replies, "No, this one feels good."
"The bridge is about to fall over," a woman appears to say, laughing.
Shortly after, in May, an internal memo on NZTA letterhead said:
"The opening and closing of the gap at deck level ... due to both pedestrian-induced vibration and strong wind events, is a significant pinch-point safety risk to pedestrians and could result in serious crushing injuries."
Walkers stumbling presented an "extremely high" risk if traffic was using other lanes, it said.
"While structural failure is not anticipated from such resonant vibrations, there is a risk that if left uncontrolled, the vibrations may lead to the box girder banging against the truss deck which could cause some local damage."
In a separate email, engineers Beca told Waka Kotahi that "any walking path" would require damping on multiple spans.
Cycle advocates engaged in a long struggle to use the harbour bridge expressed scepticism, perceiving this as another excuse for NZTA to keep walkers and cyclists off the bridge.
The video was released in an OIA response to Bike Auckland, which is lobbying for a single outer lane to be devoted to walking and cycling.
Waka Kotahi had told the group that the bridge wobbled for walkers, so it would need strengthening first, said chief biking officer Fiáin d'Leafy.
The OIA information proved devoting a single lane was no threat, she said.
"It will be safe, it will be cheap, it can be done now," said d'Leafy.
Richard Young, an independent engineer and keen cyclist who is assessing the practicality of trailing such a lane, said the gap opening and closing "could be quite alarming".
But "it doesn't look like there needs to be any strengthening work on the bridge", he said.
"The bridge only swings when there's very large numbers, we're talking thousands of people, crossing on the clip-ons."
Still, both Young and d'Leafy said the Transport Agency might do well to adopt the simple engineering fix laid out to it back in 2010 to damp down the swaying.
"They seemed to genuinely believe" it needed strengthening, d'Leafy said of NZTA.
In a 2010 investigation, Beca told Waka Kotahi that two "low-to-modest cost" options could fix the problem.
One is using large, fluid-filled plastic tanks bolted to the deck underside that interrupt the frequency, and disrupt the vibration. The tanks would be filled prior to an event.
Another option uses other dampers.
This work has not been done.
Subsequently, there have been other protest marches and cycling on the bridge.
In March this year, Waka Kotahi organised a three-day 'Walk It' event, with free tickets for 20,000 people a day to walk and bike over it, but called it off due to the storms around that time.
RNZ has asked what swaying risk mitigations it was planning on.
The May 2022 memo in the OIA said that limiting the number of walkers to 250 per span should prevent the wobbles.
The Auckland Marathon is different, as running does not set off the vibrations.
In March 2023, emails show the anti-mandate march video was spotted on social media by the bridge's maintenance team.
They sent it to Waka Kotahi, which went to its engineers.
"Any damage??" one asked the engineering firm Beca, and was told there was not.
"In future possible scenarios for pedestrian lanes on the bridge - can this be damped or managed?" they added.
The video was also sent to the team working on the second Waitematā Harbour crossing. There is no record of their response.
An announcement about the second crossing is due later this month on which of five options the government has chosen.
The second crossing is hugely problematic, having sparked several attempts over the years to get to a solution, only to end in cul-de-sacs after spending millions of dollars.
One attempt was the Skypath, and it crops up in one wobble email.
The Skypath was to have hung on the side of the harbour bridge. It got consent, then was binned.
"The lateral sway induced by synchronous pedestrian footfall at the natural frequency of the bridge has been observed on this bridge several times and is a known issue for the extension bridges," Beca emailed in March 2022.
"This was one of the key considerations for Skypath and a very high level assessment of possible damper solutions was carried out some years ago [the 2010 investigation]."
Though the bridge's swaying was little reported here, it got the attention of researchers in Britain in 2001.
Looking into the alarming swaying at London's Millennium footbridge, they noted the 1975 hīkoi's experience in Auckland.
Auckland was "particularly significant because it is a large roadbridge with a conventional structure", compared to two other smaller UK bridges they had looked at.
"In all of the above cases, the phenomenon was not fully researched or analysed, and its occurrence was not widely disseminated within the engineering profession," the research said.
It concluded synchronous lateral "excitation" could occur on other bridges with a frequency of less than 1.3 Hz and "loaded by a sufficient number of pedestrians".
Public transport advocate Bevan Woodward said yesterday that any talk of injury from swaying on Auckland Harbour Bridge was "hyperbole".
"This is just one of many, many examples by the Transport Agency to block progress on walking, cycling on the harbour bridge," he said.
