It took seven fire crews, three water tanks and heavy machinery to tackle the farmhouse fire in the small Waipango community.

And nearly half a day later, firefighters had "not even started on the inside of the house".

In just minutes, the farmhouse had burst into flames and thick layers of black smoke billowed from its roof across the rural settlement near Riverton about 11am yesterday.

Fire Risk Management Officer Murray Milne-Maresca, of Invercargill, said firefighters who came from across the region had not, late yesterday, managed to get inside the scorched property, after hours of cooling down its foundations.

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"We won't know for a while what caused the fire. We're still in the preliminary stages of the investigation because the house is very big.''

The owner of the house, who was out on the farm at the time, was alerted by his wife, who called him from the property after the smoke alarms were triggered.

Barridge Farm Partnership owner Dave Loveridge rushed to his property, where his wife was outside waiting for firefighters to arrive when he saw the blaze.

"I shot in and tried to shut the doors, but that obviously didn't work.

"I went in to try get the fire extinguisher, but it was in the room where the fire was so I just grabbed my wallet and phone and got out."

The couple's young daughter was at school when the fire broke out.

Loveridge said one of his workers came to the front door to help out, but he told him to "get out" to avoid anyone getting hurt.

The worker's wife, Mila Blasco, said she was baking when her husband came running into the house to tell her about the fire.

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She said he had told her he had tried to go into the engulfed house to "get something" but she was not sure what it was.

"He came to our house ... and his chest was sore from the smoke.

I looked out the window and saw the house with lots of flames, it was so quick."

Neighbours gathered around Loveridge offering their support and a place for him and his family to stay.

"We've got a really good neighbourhood out here, there will be plenty of beds to stay in," he said.