Dense crowds of as few as 250 people can trigger side-to-side swaying on the Auckland Harbour Bridge’s clip-on lanes – creating "significant" crush injury risks and possible damage to the structure, newly released documents show.
That risk, linked to a phenomenon known as synchronised lateral movement, has increasingly been cited in decisions to restrict or decline access for protest activity on the bridge, including a planned anti-immigration march by Brian Tamaki earlier this year.
The Auckland Harbour Bridge initially opened as a four-lane bridge in 1959, with two box girder lanes added to each side in 1969. These "clip-ons" were cantilevered off the original piers for increased traffic.
Documents proactively released under the Official Information Act showed natural vibration in the clip-ons could sync with the rhythm of walking, causing vibrations to build as groups moved together across the 1km span.
NZTA said there had been growing public interest in how the bridge was managed, alongside recent steps to "tighten up operational interventions" to protect it as a critical asset.
"Releasing these reports aims to help people understand more about the pressures facing the Auckland Harbour Bridge and what NZTA is doing to ensure the bridge remains in good condition into the future."
Footage from NZTA shows how far the gap between the clip-ons and the main bridge grew during the 2024 Hīkoi mō te Tiriti. (Source: Supplied)
On November 13, 2024, more than 5100 people walked across the west clip-on lanes of the bridge to protest the Government's Treaty Principles Bill.
A memo prepared ahead of the hikoī found the lateral motion could cause the gap at deck level between the clip-ons and the central truss bridge to open and close. The memo warned of a "significant pinch point safety risk" that "could result in serious crushing injuries".
"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 deck banging against the truss deck, which could cause some local damage," it read.
The memo also recommended limiting crowd numbers to 250 people per span to reduce "pedestrian-induced lateral movements".
As winds picked up there was swaying of the bridge during the Toitū te Tiriti hikoi. (Source: 1News)
Police and event organisers separated the crowd into smaller groups, with the largest group of around 820 people — well over the recommended limit.
Monitoring found that the clip-on swayed by up to 100mm during the hīkoi, with attendees telling 1News at the time that the structure felt "really shaky" underfoot.
These assessments have since been used to make decisions on future applications for bridge access.
A separate NZTA planning document for a pro-Palestine protest scheduled for September 13, 2025 said the protest could include about 30,000 people, and recommended no protestors be allowed to cross the bridge.
If a protest did proceed, the memo outlined monitoring and escalation triggers, including proposals to monitor lateral displacement using laser pointers, and a stop trigger if displacement continued to increase once structures began colliding.
The document further recommended restricting access in high winds, noting wind combined with large crowds would reduce safety margins. High winds ultimately resulted in the protest being rerouted.
An anti immigration march organised by Brian Tamaki for January 31, 2026 was declined permission to cross the bridge after NZTA said it failed to meet the high threshold required for special access to what it described as New Zealand’s most critical transport asset.
An NZTA spokesperson told 1News in a statement that the agency had a responsibility to protect both users of the bridge and the integrity of the structure.
"[This] is why we are increasingly restricting access to events causing pedestrian induced vibrations, which the bridge was not designed to withstand.
"NZTA does not allow any unpermitted activity that does not meet strict criteria for access, including protest activity, across the Auckland Harbour Bridge."
The agency said it had a formal approval process for all activities on the state highway network, including the bridge, requiring detailed safety and traffic management plans and risk assessments funded by organisers.
Events such as the Auckland Marathon were permitted because runners moved in a dispersed and dynamic pattern, reducing risk of synchronised lateral movement on the clip-ons.
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