An investigation into the collapse of a power pylon in Northland that cut power to most of the region has found contractor error was to blame.

Omexom was carrying out routine maintenance to the baseplate of Tower 130, a Transpower transmission tower servicing 100,000 properties, when the pylon fell on 20 June.

The report, released on Thursday morning, said a relatively inexperienced and inadequately supervised worker removed nuts from three of the four legs of the pylon, compromising its stability and causing it to rupture and fall.

And it was not the first time nuts and washers were removed from more than one leg of a pylon at once, which did not align with the contractor’s own standard practice.

At the time the government called it “completely unacceptable” with the cost of the widespread power cut estimated at $60 million.

The outage as it happened.
Photo: Supplied / Top Energy

In the 73-page report, investigator Daniel Twigg said Omexom taught its civil works field staff to remove all nuts from only one tower foundation leg at a time.

A large power pylon has come down.
Photo: Supplied / Top Energy

“This standard practice, which was taught to the team leader at Tower 130 during 2023, was not followed at Tower 130 on 20 June 2024.”

Instead, the team leader was sandblasting while the “relatively inexperienced” team member was assigned the task of removing hold-down nuts from the legs, as the sandblasting work moved from leg to leg.

During the work the team member removed nuts above the baseplate from three tower legs.

“These actions did not comply with Transpower’s specifications and service standards and did not meet standards of ‘good industry practice’.

“The team member who removed the nuts from the foundation legs was not adequately trained to perform the task and was not adequately supervised while performing the task.”

Twigg said Omexom completed four baseplate refurbishments in June before moving to Tower 130 near Glorit, off State Highway 16.

“At earlier jobs in June, the evidence suggests that nuts were removed from more than one tower foundation leg at a time.”

Tower 130 is a 220 kilovolt (kV) transmission tower on the Henderson to Marsden A line, carrying two 220kV circuits between the Northland region and the remainder of the North Island.

At the time of the maintenance, one of the 220kV circuits attached to Tower 130 was on a planned outage to allow maintenance work to be undertaken to that circuit.

Shops across Northland were closed for business.
Photo: RNZ / Peter de Graaf

Engineering consultancy firm Beca was commissioned by Transpower to investigate why Tower 130 fell and concluded the cause of its failure was the removal of nuts from legs A and B by the Omexom field crew, Twigg said.

Those nuts were intended to resist tension forces on the tower and their removal ultimately caused the tower to fall.

Transpower stopped all baseplate work across the country on 21 June and these will not be restarted until all reviews of the incident had been completed, Twigg wrote.

He recommended that before any baseplate work resumed, Transpower should review and revise its drawings and specifications to ensure a description of nut removal and when engineering advice should be sought.

At the time of the incident, Transpower did not detail this description for baseplate refurbishment or specify how many nuts should be removed at the same time or in what order.

Omexom also did not detail in its written procedure how many nuts could be removed at the same time though its standard practice was from only one leg.

Twigg also recommended service providers to produce new work method’s for baseplate refurbishment for Transpower’s review.

And that Transpower hold refresher workshops with service providers.

In a statement issued ahead of a media conference by Transpower, it said it accepted the investigation’s findings and recommendations and was already moving to address them.

“We can’t undo what happened, but we can put in place improved measures to prevent anything like this happening again,” Transpower acting chief executive John Clarke said.

“Our specifications require that nothing is done that could compromise tower stability. We don’t prescribe how many nuts should be removed and in what order, as service providers have the flexibility to determine their own processes that achieve the task safely and effectively.”

All baseplate refurbishment work was stopped within 24 hours of the pylon falling and would not be resumed until the recommendations were in place, Clarke said.

During the media conference, Clarke said what happened in Northland was “so far outside sensible practice it’s difficult to believe it could happen”.

Clarke said he could not comprehend how Omexom assigned a crew without sufficient training and supervision to the job.

Clarke said he was awaiting the result of Omexom’s own review into the incident which was expected later this month.

Transpower grid delivery manager Mark Ryall told the media the work order Transpower approved indicated there would be two experienced workers and a supervisor on the job which did not happen.

Ryall said Transpower had had a 30 year relationship with Omexom, and audits showed they did have quality systems in place and from that level of information Transpower did not see a systematic failure.

Something on this job went “horribly wrong” and Transpower was working with Omexom to see what happened there, he said.

Transpower spent between $150m and $200m a year maintaining towers across the country including about 200 baseplate repairs and maintenance.

In a statement, Omexom managing director Mornez Green apologised and said the company had been doing the work for decades without problems.

Our standard practice was not adhered to – this cannot happen ever again,” he said.

“No work should ever be done that compromises tower stability.”