On a scorching, cloudless summer's day, it was the perfect storm.

A tractor driver ploughs a stony, tinder-dry paddock while the mercury tops 28C. Tall, parched grass. A blustery, warm wind with low humidity and an "extreme" fire danger.

The result was a "Swiss cheese effect", where all of the holes managed to align, and the sparking of New Zealand's largest wildfire in more than 70 years.

February's great Pigeon Valley blaze, 30km south of Nelson, would burn out of control for days across 35km, destroy or damage 2300ha of pine forest, raze a three-bedroom home, multiple outbuildings, fences, shelter belts, pastures, and cause the evacuation of an entire town of around 2500 people.

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A final report into the origin and cause of the fire, released today, reveals for the first time how the contractor's disc plough had struck stones at least 18 times before the fatal spark. Newly-released photographs show rocky charred ground that looks more like a riverbed than a farmer's field.

And yet, it's been declared an unfortunate accident.

"This fire proves that, in extreme weather conditions, a seemingly everyday rural activity can end up causing widespread damage," Fire and Emergency NZ (FENZ) principal rural fire officer for Nelson-Tasman Ian Reade says.

"It was the proverbial perfect storm."

The investigation begins

Jamie Cowan got the call about 7pm on February 5 this year.

There'd been a big fire start up near Nelson and would the Queenstown-based experienced wildfire expert, who worked on the 2016 Port Hills fires, carry out an investigation?

He was on his way.

Hydraulic technician Joel Scott was driving back from the forestry block down Pigeon Valley Rd earlier that afternoon when he spotted leaping flames.

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Ditching his ute in the middle of the road, he ran towards the fire and made the first 111 call.

"We're going to need a chopper real fast," he tells the call operator at 2.14pm.

Scott took the first photo of the inferno and approached a "flustered" man at the scene. He nearly gave him "a mouthful for starting the fire or having a fire going" after six weeks of little to no rain.

The first flames from a spark caused by disc ploughing on the Pigeon Valley farm. Photo / Supplied
The first flames from a spark caused by disc ploughing on the Pigeon Valley farm. Photo / Supplied

It turns out he was the tractor driver. He'd tried to phone 111 himself but couldn't get a cellphone signal and was watching on helpless.

He'd been discing – towing a plough behind a large Fendt tractor – since early that morning on land leased by local farmer Ian Parkes.

After churning over several neighbouring paddocks, the contractor had started on the stony area near a creek.

When he showed up at the charred, blackened ground the next day, fire investigator Cowan was stunned at just how rocky it was.

"From my previous experience working in the agriculture/horticulture industries, I was surprised it had been disced it was so rocky," Cowan says in his detailed 116-page report.

Hunched over, scouring the scorched earth, he found multiple "strike marks" on rocks where the metal discs had passed. Often, metal had been "smeared" onto the rocks.

Metal smeared onto rocks by the discs outside the origin area. Photo / Supplied
Metal smeared onto rocks by the discs outside the origin area. Photo / Supplied

There were 18 "significant stone strikes" found in total. Magnet tests around the strikes showed lots of metallic filings and particles.

Later, when the contracting company boss was asked how common it was for the plough's 'scrapers' – metal blades which keep the discs free of debris – to come into contact with the discs when going over rough ground, he replied, "Very common".

"I suggest they get every helicopter they can"

The tractor driver told Cowan how he'd been discing for just a few minutes when he noticed smoke coming from an area he'd just ploughed "around two minutes before".

He desperately tried to put the fire out by discing over it. When that didn't work, he grabbed a fire extinguisher. But by then, the fire was too hot and increasing in size every second.

Scott said that within minutes, fuelled by southerly winds, it had gone from "the size of a barbecue fire" to galloping up and over a steep and recently harvested hillside of forestry, into a vast plantation block.

Wakefield Volunteer Fire Brigade, at the very bottom of Pigeon Valley Rd, were alerted to a vegetation fire at 2.16pm, according to the Fire and Emergency NZ (Fenz) SMS incident report released to the Herald under the Official Information Act legislation. Brightwater units were roused about the same time.

The first Wakefield fire truck got there at 2.26pm, followed by a Brightwater appliance at 2.34pm.

And the emergency calls kept coming – 13 in 22 minutes. There would be 27 calls in total.

Fire and Emergency NZ's internal message log first records details of a bush fire at 2.15pm. One minute later, it notifies of "a large bush fire – spreadin [sic] due to winds".

By 2.20pm, a fire lookout operator stationed on the Barnicoat Range above nearby Richmond has directed the first helicopter to respond.

The first helicopter arrived on the scene at 2.40pm, Fenz says. Two more arrived at 2.45pm.

But even though it took just 26 minutes to scramble a chopper and for it to arrive, the blaze was already out of control.

"I suggest they get every helicopter they can get their hands on, now," says a frantic Brightwater farmer calling 111 at 2.48pm.

By 6.33pm, the fire was "uncontained", with five helicopters attacking it from the air.

Residents in Eves Valley and Teapot Valley were by now being evacuated.

Ground crews pulled out of Eves Valley shortly after 8pm "and leaving it to the helicopters as it is too dangerous". But by about then it was getting too dark for the helicopters to safely operate.

And as night fell, it was descending on Redwood Valley, sending fire appliances to retreat to the police cordons.

A three-bedroom house in the upmarket Redwood Valley with panoramic north-facing views down to the Nelson bays was burned to the ground.

The owner of the engulfed home feared his multimillion-dollar Spanish-style mansion just metres away had also gone up in flames.

That night, he stood at the bottom of his hill watching flames "very quickly come over the ridge and move down" towards his property.

"It was a line of fire, just swiping down," said the homeowner who only wanted to be known by his first name, Michael.

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Soon afterwards, there was an "explosion" which he assumed was his cottage "going up in flames".

Around midnight, he got an alert from his security alarm company saying the property was on fire.

"It was absolutely devastating," Michael said.

By the next morning, Pigeon Valley was deemed "safe", but residents still weren't allowed home.

Some landowners were escorted back to grab pets and transport livestock.

One farmer at Redwood Valley lost more than 60 sheep.

"It was pretty horrible, to say the least," said agricultural contractor, TB tester and part-time farmer Steve German.

That afternoon the Redwood Valley fire was contained around the houses.

Sixteen aircraft were available for the aerial assault. Six more were available by day two, along with two fixed-wing aircraft which were used to drop fire-retardant.

Over the following days, a total of 61 units from across the country were scrambled to join the fight.

It would be the biggest ever aerial firefighting operation in New Zealand's history.

The blaze spread across 2316ha, spanning 36.4km – a greater area than the devastating 2017 Port Hills fires in Christchurch.

After 22 days, Civil Defence lifted the state of emergency.

The findings

On the second day of his probe, Cowan and a mechanic took a look at the tractor and disc plough.

They couldn't find any obvious damage or mechanical faults with the machinery, and work logs and maintenance records stacked up. And Cowan didn't find any birds' nests or build-up of grass under the tractor bonnet which could have ignited either.

The Fendt tractor that was pulling discs at the time of ignition. Photo / Supplied
The Fendt tractor that was pulling discs at the time of ignition. Photo / Supplied

The contracting firm owner also confirmed his driver didn't smoke cigarettes, and none had been found at the scene. Nor was there any evidence or other fires being lit, lightning strikes, powerlines in the area, or traces of arson or any other suspicious behaviour.

From his scene examinations, witness statements, photos, and other information hoovered up in his investigation, Cowan turned his attention to the fire's cause.

He came up with four possible scenarios but soon ruled out two: build-up of grass or fine fuels under the tractor's bonnet or around the exhaust; and scrapers coming in contact with the discs created a hot surface that fuel has been trapped/rested against and caused ignited the fire.

In the end, he came to the most likely hypothesis that the discs struck a stone, or rubbed on the scrapers, and "resulted in a spark(s) or a hot surface that has contacted and ignited the fine fuels".

Cowan concluded that the fire should be classified as "Accidental"and caused by "Equipment Use".

"I am confident this fire was caused by the operation of the discs in very stony ground on a hot, very dry windy day," he says.

"To determine if it was a spark from the disc or scraper, or the hot surface created when the disc and scraper have come together is a question I cannot answer. I believe this report has proven both to be competent ignition sources. Combined with the 'Swiss cheese effect' of perfect weather and fuel conditions, a fire from a source not historically considered a fire risk has occurred."

Disc showing wear and tear on cutting edge. Photo / Supplied
Disc showing wear and tear on cutting edge. Photo / Supplied

New Zealand Police last month said they had not pursued any criminal charges.

"The cause of the original Pigeon Valley fire on February 5 was investigated at the time, and the cause was determined by fire investigators. As a result, police were not asked to investigate that fire any further," said a spokeswoman who directed further enquiries to Fenz

An independent operational review into Fenz's response to the Tasman fires, which includes the likely-deliberately lit Atawhai and Rabbit Island blazes, is still ongoing. Fenz hopes to publicly release its findings in October.

A spokesman today said Fenz will review the findings of the fire investigation report, released today, in the coming months to "determine if any further action is required".

Insurance Council of New Zealand chief executive Tim Grafton said while he did not know how insurers will respond to Pigeon Valley, in general terms, insurers reserve the right to "pursue recovery from at fault parties and/or their insurers".

Cowan concludes his report with some recommendations on how to prevent future disasters.

While he says discing should be considered "a low risk for potential ignitions in most circumstances", when done under extreme fire conditions it should be subject to best practice "trigger points" like those used in the forestry industry, which identifies when the fire risk is high and operations should stop or fire risk mitigating factors employed.

Other basic precautions, Cowan says, should include discing a break around the outside of a paddock to prevent fire spread if an ignition does occur, discing rocky ground in the morning and having larger fire extinguishers or another portable water source on hand during extreme fire days. Further mitigation could include modifying discs so that the scrapers cannot contact the discs or manufacturing them out of a different compound that will not cause sparks.

Local principal rural fire officer Reade said while the conditions Nelson and Tasman experienced last summer were extreme, they were not unique. Parts of Marlborough, Canterbury, Otago, Bay of Plenty and Northland all had similar fire risk profiles and with climate change continuing to heat up New Zealand summers, it is likely the dry summer conditions will become increasingly common.

"It's important people take heed of our advice, which focuses on reducing the likelihood of fires starting, and mitigating the potential damage to property if a fire occurs," he said.

"There are many simple things people can do to prevent fires starting, from not using machinery which can emit sparks in hot, dry and windy conditions, to ensuring electric fences are not in a position or used where they could spark onto dry vegetation."

And Fenz says it's already reviewing its guidance to the community for the coming summer fire season and looking at how it can reach more people more effectively. In the Nelson Tasman area, it's working with a range of sectors, including forestry, farmers, rural contractors, engineering companies, and roading contractors.