No one will be prosected over the pipeline rupture that sparked a major fuel shortage last year, mainly because authorities do not know who did it.

The pipeline break disrupted supplies from the Northland refinery and caused dozens of flights to and from Auckland Airport to be cancelled.

Two digger drivers were interviewed about working at the Ruakaka site when Refining NZ's Auckland fuel products pipeline was ruptured, but neither could recall working near the pipeline.

Northland Regional Council group manager of regulatory services Colin Dall said the fuel leak - estimated at 124 cubic metres – was a breach of the Resource Management Act but the council "does not have a case for prosecution".

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Dall said lengthy investigations found an unknown digger illegally searching for swamp kauri "may have" operated around the pipeline at the Ruakaka property.

First responders to the ruptured pipe in September.
First responders to the ruptured pipe in September.

Gouges apparently caused by a digger were believed to have triggered the pressurised pipeline's failure, but the actual date of the damage and what did it were not known.

The damage could have happened any time after the pipeline's last inspection with a "magnetic flux leakage intelligent and sizing calliper pig" in July 2014, he said.

Aerial imagery from late 2014 appeared to show lightly disturbed earth at the rupture site but "it is only with the benefit of hindsight that the image can be taken to indicate that there may have been deeper digging for swamp kauri that has subsequently been covered up".

The site owner told the council he had been approached in late 2014 by a contractor speculating for a kauri wood company asking for permission to "scratch around and see if there were any logs".

The owner said he had given the contractor, whose name and contact details he did not know, "limited permission", pointing out an easement indicated with white posts and telling him there was a pipeline there.

The owner says his "clear intention" was that the contractor should not dig within the easement area and Dall said legally, the owner could establish that in the circumstances he had taken "all reasonable steps" to prevent an offence. Warning signs were up at the site.

Dall said the council had found invoices provided by a transport company for delivery and pick up of a digger at the site in August and October 2014.

Two digger drivers were subsequently interviewed. One said he could not recall doing any work in Ruakaka and the other said he had driven a digger at the front of the site for one day – but not around the pipeline.

Legal advice obtained by the council was that the information it had about the site owner, the digger drivers and the digger owner fell well short of the standard of proof required to prosecute.

Dall said the council's investigation had also determined neither Refining NZ, which owns the pipeline, nor First Gas, which monitors and inspects it on on the refinery's behalf, had any role in causing the leak.

''The discharge was beyond the control of either party or their employees, both of which had a suite of very extensive protective measures, checks and other precautions to prevent damage to the pipeline.''