Refining NZ is urgently trying to fix a jet fuel line in rural Northland that has almost grounded the country's aviation network.

An "external force" in a Northland field led to Auckland Airport's primary jet fuel line being damaged, but local residents are adamant the land was only used for grazing.

New Zealand's aviation network has been put under enormous strain after the pipeline that connects Marsden Oil Refinery and Auckland Airport suffered a major leak.

Scores of flights have been cancelled because of the fuel shortage, and thousands of international and domestic air passengers have faced disruption as a result.

CHRIS MCKEEN/STUFF The pipeline which connects Marsden Point Oil Refinery and Auckland Airport was breached on a farm near Ruakaka.

But locals insist kauri swamp digging is not to blame for the debacle.

READ MORE:

* List of flights cancelled due to Auckland Airport jet fuel supply shortage

* MPs lament jet fuel crisis

* Fuel pipeline cut 'outside our control', says Air NZ

* Auckland's fuel supply vulnerability is well known, and serious

Local resident Allan Richards, who lives across the road from the leak, said paddocks above the pipeline were "just grazing land" for sheep, cattle, and horses.

CHRIS MCKEEN/STUFF The site is around 8km south of Marsden Point Oil Refinery and close to Ruakaka.

Richards says he has lived there since 1981 and has never seen kauri excavation taking place on the paddock.

He said he hadn't seen any heavy machinery there until pipeline repairs got underway on Thursday afternoon.

Richards said he first smelled the leak — "like a mixture of petrol, diesel, and kerosene" — on Friday morning.

CHRIS MCKEEN/STUFF People hit pipes all the time in New Zealand, but usually the consequences are more limited.

On Tuesday afternoon, Refining New Zealand said efforts to repair the pipe were "progressing well" and that jet fuel delivery into Wiri was expected between midday on September 24 and midday on September 26.

From that point, it would take another 30 hours before fuel could be transferred to the airport as it needs to settle and be recertified, the company said.

The Northland Regional Council said Refining New Zealand could be prosecuted over the debacle, and was investigating the circumstances that led up to a fuel pipe line from a Northland refinery being temporarily shut down.

CHRIS MCKEEN/STUFF The damage to the jet fuel line has put enormous strain on the country's aviation network.

The 168-kilometre pipeline – which carries jet fuel, petrol and diesel directly from the oil refinery at Marsden Point in Northland to tanks in Wiri, south Auckland – has been out of action since Thursday afternoon, when workers at Refining New Zealand noticed a drop in pressure on the line.

A jet fuel leak was located on a rural property 8km south of the refinery later that afternoon.

Refining New Zealand spokesman Greg McNeill said on Monday initial investigations showed a digger "scraped" the pipe.

STUFF Refining NZ has said it will reprocess any jet fuel it can recover at Marsden Point Oil Refinery.

The damage had caused corrosion and ultimately "a small tear" on the pipeline, McNeill said.

"Clearly we've got an issue where some external force has gone and dug up the pipe."

The regional council said, "as with any spill", it would investigate the circumstances leading up to it.

NICOLE LAWTON/STUFF Dozens of domestic and international flights were cancelled over the weekend and into Monday due to the fuel shortage.

"This could include formal enforcement, including potential prosecution under the [Resource Management Act]."

Prime Minister Bill English said that a contingency for this type of incident had been previously been looked into but wasn't economically viable.

"There have been a couple of studies done that looked at different alternatives for backing up the current infrastructure, and the decisions were made that the investment that would be required to double up would be too much to be passed onto consumers.

"But I expect that after this that they'll go back and have another look at it.

"In the meantime, the oil companies, the airlines, the agencies, are all working together, we've offered all the support, any support that they need, they haven't actually made any requests yet," English added.

In a Facebook post shared on Thursday, Refining NZ revealed it had "spotted a leak of jet fuel on a section of the Refinery Auckland pipeline near Ruakaka".

It added it was working with oil companies to ensure that fuel supplies weren't "unduly impacted".

In a later post, Refining NZ said on Friday that it had "successfully contained" the leak and was busy clearing the area.

It said it had been liaising with a number of families in the vicinity, believed to be on the southern stretch of Marsden Point Rd, and had provided alternative accommodation where needed.

McNeill could not confirm whether it was a kauri line digger which cut the line, but did say kauri had been excavated in the area surrounding it.

In-house experts were called in to investigate the damage and found scratchmarks on the piping, McNeill added.

Adding to the problem was the fact that the surrounding soils were very acidic, advancing the corrosion rate, he said.

McNeill initially said a fix could take up to two weeks because of the difficult access to the site near Ruakaka where the pipeline had been cut, and the need to have repairs properly certified.

"We have got heavy equipment in there and it is very soft peaty soil."

The fix would need to be certified by a Lloyds' pipeline expert, he said. "The key thing here is safety of operations."

It was likely Refining NZ would first operate the pipeline at 70 per cent capacity until it knew the repairs had been successful, he said.

The timeframe of 10 to 14 days was just an estimate, he said.

"The really positive thing is we have made great progress."

While workers had secured a replacement pipe, they first needed to ensure all jet fuel was removed from the ground to ensure the area was safe.

McNeill said they believed it would be fixed between September 24-26.

"Getting the product down to Wiri where it is stored is the key to us, we're working toward a definite timeline," he said.

Once the jet fuel is back at Wiri it will undergo about 30 hours of processing before it is certified and taken on to the airport.

The fuel pipelines are checked annually, and this particular line was "completely clean for any corrosion" at last inspection, he said.

Airport chief executive Adrian Littlewood said some 27 domestic and international flights were cancelled over the weekend, while others travelled via other airports to refuel.

Those affected from Monday morning would include international passengers on flights to Sydney and Melbourne and domestic travellers to most of the main centres.