A rupture to the pipeline taking fuel from Marsden Pt Refinery in September caused major disruptions, leading to fuel shortages and flights being cancelled.

But despite evidence of a digger having hit the line not far from the refinery in Ruakaka, nobody will be prosecuted for the rupture - mainly because authorities can't identify who was responsible.

Northland Regional Council said after a lengthy investigation it cannot prosecute anyone for the mid-September fuel leak from the pipeline.

NRC Group Manager – Regulatory Services Colin Dall said while the discharge itself – estimated at 124cu m – was a breach of the Resource Management Act, the council "does not have a case for prosecution of any party".

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

Gouges apparently caused by a digger were believed to have triggered the failure of the pressurised pipeline, but the actual date of the damage and its specific cause were not known.

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

Refining NZ welcomed the NRC finding that it had no causative role in the pipeline rupture, as well as its commendation for the refinery's outstanding response.

NRC found that the discharge was beyond the control of Refining NZ which had a suite of extensive protective measures, checks and other precautions to prevent damage to the pipeline.

NRC also concluded Refining NZ could not reasonably have foreseen or provided against the damage.

Refining NZ chief executive Sjoerd Post said it was pleasing that after reviewing the external expert reports and having witnessed first-hand its containment and recovery processes, the council had decided not to prosecute the company.

Mr Dall said the landowner where the rupture occurred said that in late 2014 he had been approached by a contractor speculating for a kauri wood company asking for permission to "scratch around and see if there were any logs".

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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 said his "clear intention" was that the contractor should not be digging within the easement area and Mr Dall said legally, the owner could establish that in the circumstances he had taken "all reasonable steps" to prevent an offence. There were also warning signs at the site.

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

Mr Dall said for evidential purposes, authorities had no legally credible evidence the digger involved had operated around the pipeline itself, or that either of the two digger drivers had worked that pipeline area.