Fire and Emergency NZ (Fenz) incident controller John Sutton said it was almost certain the fire was the result of agricultural machinery.
The owner was "mortified", said Sutton, who added it was a highly accidental event.
"A totally unintended consequence really," he said.
Sources told the Herald yesterday that the fire was sparked by a farmer tilling dry fields at around 2pm up Pigeon Valley near Wakefield, about 30km south of Nelson.
Sutton, in the latest update on the 1900-hectare blaze, said he had an overflight of the area and it had been a big multi-agency response.
"It's sad to hear about losing one house but I have to put that in perspective."
The fire had got so close to other houses that paint had burnt on them, he said.
While he still described the fire as "out of control" it did not progress yesterday due to weather conditions, he said
Certainly, by the end of today he'd like to say there is a measure of containment, he said.
Twenty-two helicopters are now available for fight the inferno, he said.
"We have a big day on today - fortunately, the weather is in our favour."
Some firefighters were experiencing fatigue and they were trying to fly in more firefighters, he said.
The fire had spread to 1900 hectares at two sites near Nelson — about four times the size of Auckland's CBD and a greater area than the devastating Port Hills fires in Christchurch two years ago.
About 235 homes had been evacuated, mostly in the Pigeon Valley region, and residents were told they would not be able to return for two or three days.
RNZ reports at least one home has been confirmed as destroyed by the blaze.
The Nelson community has rallied around to help those forced to flee the flames, with offers of accommodation, hay for affected animals and feeding volunteers.
They were not able to take things from the house, he said
Kempthorne is one of many people whose properties have been evacuated.
He told Newstalk ZB's Kate Hawkesby it has been an anxious time for everyone who has had to leave their homes.
"If you're in a rural-residential area, the fire is burning, and it's burning a lot quicker than people thought, and then suddenly it's on the top of a hill behind your place, it is very scary for people to have to leave."
Kempthorne says fire crews are making good progress.
"Fire crews are doing an incredible job, working through the night. With a change of wind direction and a cooler day, it has made quite a difference."
However, crews are still on high alert. Smouldering trees could reignite if conditions worsen.
"That's what they're going to be working on with helicopters and ground crew -- just gradually getting on top of those areas."
PM to assess damage today
Prime Minister Jacinda Ardern will fly in today to survey the damage from the fire.
The Defence Force was called in to provide reinforcements for firefighters who had been battling the blaze for more than 30 hours. Nearly 50 firefighters were supported by 14 aircraft, which stopped flying yesterday evening while ground crews kept working through the night to contain the blaze.
Some residents have had lucky escapes.
Redwood Valley Rd resident Graeme Sutton was reluctantly evacuated on Tuesday night after noticing a "great plume of smoke ... shading the whole city".
"One or two farmers didn't want to leave," he said. "If I had a choice I would have stayed. You want to be there to defend it."
His family were grateful their house had been spared, he said. They had packed photos among their belongings in case they lost their home.
Tasman deputy mayor Tim King fled his home of 48 years as smoke crept closer, and he fully expected it to burn to the ground. Firefighters doused the fire just 5m from his doorstep.
"The fire crews did an absolutely amazing job," he said.
David Horncastle lives in Pigeon Hill and his family home is next to a Carter Holt Harvey tree plantation.
"Last night the flames were really, really high. The wind was coming straight at our house but changed direction."
He recruited a friend to make a fire break with a digger in case the flames swung back at his property. They left the property again last night, uncertain whether it would be safe.
"At 8pm the helicopters get parked up, they can't fly at night and don't start again until 6am in the morning. If any fires spark up in the night then we are on our own so we are also evacuating."
The fire's growth slowed yesterday afternoon and fire crews were confident it could be further contained as long as the wind did not pick up again. A second fire which broke out at Rabbit Island near Nelson yesterday afternoon spread over 10 hectares before being contained.
There was unlikely to be any help from the weather because no further rain was forecast for the bone-dry region, which has seen barely any rain in 40 days.
MetService meteorologist Rob Kerr said the area could get some rainfall on Sunday but the front heading there could lose its power before reaching Tasman.
He said January had been "remarkably" drier than last year. Only 6mm of rain had fallen last month, down from last year's total of 220mm when two tropical cyclones battered the country with rain and wind.
