"It's also taken out two sheds."
While an investigation into how the fire began was still under way, Pyatt said it appeared the tractor might have caught a tyre lying in the grass, causing a spark to ignite the machine and grass.
The fire, fanned by the strong northwesterly winds, had spread quickly and neighbours had self-evacuated, he said.
"The surrounding houses had good clear space free of trees."
About 40 crew and eight vehicles attended the blaze and Pyatt commended the efforts of everyone involved, including the speed with which fire crews arrived at the scene.
"Initial efforts by the Carterton and Greytown volunteers, they literally saved property here today," he said.
"This is a really busy time of year for us … we continue asking our volunteers to stand up to put out fires day after day."
He urged those who would garden with machinery to think carefully about the conditions.
"We're pleading with the public to be really, really careful," he said.
"We would urge everyone in Wairarapa to be mindful about the hot, dry, and windy conditions."
Mandy Bedingfield and partner Raymond Hicks were contracted to look after the land, which was owned by a property developer based in Upper Hutt.
Bedingfield said Hicks had mown the lawn a couple of weeks ago and was just there to tidy up some of the edges on Thursday afternoon.
"He hit an old wheel that was in the grass," she said.
"He jumped out of the tractor, thinking that he could stamp it out."
She acknowledged the speedy arrival and sincere efforts of the fire crews.
"They did get here really quickly," she said.
"It's unfortunate, but at least everybody's okay.
"We'll be dropping them some beers off at the station."
Costley St resident Thomas Beagle was at home when the fire started.
He had been on the phone when he saw the fire engines drive into the vacant lot behind his house.
"I ran out the back, and there were lots of flames and smoke in my backyard," he said.
"It was travelling at speed."
The fire had burnt some of Beagle's back section, stopping where he had cut his lawn.
"You can see how a mown lawn is well worth it," he said.
"We've been worried about it before."
A Brooklyn Rd resident, who did not wish to be named, said he had heard the tractor before the fire started.
"We could hear the mower hitting the stones from inside the house."
An area in the middle had been left unmowed when the field had been mown a couple of weeks ago, he said.
"He's probably been told to tidy it up," said the man, who had been a dairy farmer for 15 years.
"I've spent hundreds of hours on a tractor mowing hay," he said
"You mitigate the risk, you don't then go out on a 28 degree day … You do not mow in this type of weather."
He said it should have been mown earlier as there was no stock grazing on the land.
"There is no reason why they should be doing this at this time of year," he said.
"This has just been a disaster waiting to happen."
The fire crews' actions had saved neighbouring houses as they had left the tractor to burn and instead focused on containing the radius of the fire, he said.
"The tractor was the sacrificial lamb," he said.
"I think the brigade did a very good job concentrating on the houses.
"The flames were as high as the houses.
"The smoke was going through our neighbour's window, and she was trying to get her cat and dog to get them out."
