A report by Refining New Zealand has shed more light on the Marsden pipeline rupture that stopped fuel supplies to Auckland in September.
Experts have concluded the pipe was weakened after it was scraped by a digger at some point in the past three years. It burst open as the refinery gradually increased pressure in the pipe to meet increased demand for fuel.
The report, to the Northland Regional and Whangarei District councils, details the events leading up to the pipeline break at Ruakaka, 8km south of the refinery, on 14 September.
The company engaged global engineering company Quest Integrity to analyse the cause of the rupture and sent it the damaged piece of pipeline for forensic tests.
Quest concluded there was nothing wrong with the way the pipe had been maintained.
Records showed it was undamaged in 2014 when it was last inspected internally by an electronic device that runs through the entire length of the pipe and minutely examines its interior.
But Quest said the section that ruptured in September had been badly gouged and scraped since that time and had begun to crack before it finally burst open.
"The pipeline failed ... while operating within its certified pressure range as a result of overload from one of the pre-existing gouges that was aligned axially along the pipe. The gouges had the effect of concentrating the hoop stress in the pipe making it prone to fracture," its report said.
Earlier this year, Refining New Zealand began pushing more fuel through the pipe, to meet a growing demand from Auckland markets.
The pipeline is certified by Lloyds Register to handle internal pressure levels of 90 bargs.
It ran at 87 bargs until 1999, when the refinery installed new pumping stations allowing it to drop the pressure to 75 bargs with no reduction in fuel volumes.
Before it stepped up the pressure again this winter, to meet demand, it brought in global pipeline experts Worley Parsons from Canada to oversee the process.
The company recommended some equipment upgrades and a stepped process of increasing the pressure gradually over six weeks from June to August.
By the time the pipeline ruptured in September it had been running at its old level - 87 bargs - for about 10 weeks.
Quest Integrity concluded the direct cause of the rupture was damage to the pipe caused by a digger.
Photographs of the inside of the pipe showed the gouges, like ribs, running along and around it.
The report concludes: "If the gouges were not present in the pipe, there's no reason to believe the pipe could not ... be operated safely up to the specified maximum allowable operating pressure of 90 barg."
Since the pipeline rupture, Refining New Zealand has found aerial imagery of the site showing slightly disturbed earth, and several kauri logs in the vicinity.
Its monthly flyovers of the pipeline spotted nothing unusual at the time; the disturbance looked similar to nearby tilled fields.
It said with the benefit of hindsight, there may have been deep digging for swamp karui that was then covered up.
The Northland Regional Council has spoken to the owner of the land where the pipeline broke.
He has confirmed there was digging on his property but refused to identify the digger driver.
The council doesn't have the authority to make him do so.
Council chair Bill Shepherd said the council had no powers under the Resource Management Act to compel anyone to provide information, making it difficult to prosecute anyone for the spillage of 124 cubic metres of jetfuel.
Mr Shepherd said that lack of power was something he'd like the forthcoming government inquiry to address.
"It does tie our hands, pretty much, in carrying out a full in-depth investigation of what caused the pipe to rupture so it would be really good if the inquiry could look at those aspects of the RMA."
The council said Refining New Zealand did a good job of containing the fuel spill and mopping up afterwards, and the company's insurers, Lloyds, agreed.
In the refinery's report, an insurance inspector describes the Refining New Zealand's response to the pipeline break as exemplary.
"The organisation and works undertaken on site are the best I have seen in over 20 years of dealing with environmental pollution events," the inspector said.
Energy Minister Megan Woods has said the inquiry in the new year into the pipeline rupture will get to the bottom of who was responsible and examine the security of the country's fuel supply.