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Spontaneous combustion has been blamed for the loss of more than $40,000 worth of hay on a Patearoa farm early yesterday.
The Ranfurly Volunteer Fire Brigade was called to the fire about 6.15am, after "600 to 700" bales of hay went up in flames, Chief Fire Officer James Hazlett said.
He said the bales had been there "about three to four weeks" and may have still been green.
"If the hay has too much moisture ... if it is too green, it heats up."
He said the brigade could not extinguish the fire and after three hours' work left the landowners to monitor it. Because it was outside the brigade's district, the rural fire crew were notified and they turned it over to the Department of Conservation (Doc) as it was within 1km of a Doc reserve.
The fire was still burning when the Otago Daily Times spoke to Mr Hazlett at 5pm.
Doc Alexandra rural fire officer John Keene said they had "basically just left it up to the farmer to keep an eye on".
He said it was not likely to flare up again but "if it does, it will be contained". The Ranfurly brigade had set up a water supply there.
Mr Hazlett said such fires were rare - there had not been one in the area for about 10 years.
The hay was insured.