The fire broke out shortly before midnight on Wednesday and destroyed two haysheds, containing 12,000 to 14,000 bales, at Andrew Quigley Contracting in Winslow.
Fire and Emergency New Zealand incident controller Don Geddes said the blaze was brought under control by about noon on Thursday.
Geddes said three crews are working with contractors today to continue dampening down the burning hay before burying the residue.
"We have one fire truck, two tankers, and 14 people from Fire and Emergency, and seven contractor staff with a bulldozer, three diggers, a water tanker, and two dump trucks working on the fire," he said.
"We’ll be digging pits to bury the residue, which will be wetted down and eventually capped with shingle two metres deep to make it safe.
"It’s a huge job, so we’re expecting to be working on this 24/7 until the end of the weekend."
Some of the hay is still burning and the cause of the fire has not yet been determined, Geddes said.
He said smoke from the fire is likely to drift from a north-west direction from the site today and people in the area should keep their doors and windows shut.
Yesterday, Ashburton district councillor Rob Mackle said he saw smoke coming from the property.
He told RNZ Quigley was well-respected in Ashburton and the farming community.
"It's a large contracting business, employs a number of staff, and they will be out there helping Andrew at the moment," she said.
"There'll be a massive clean-up for him going ahead, but it is a great community here, they'll all be pitching in to help I imagine."
Ashburton Mayor Neil Brown was at the scene working a day shift for the fire brigade, Mackle told RNZ.
He said he assumed the fire was caused by combustion, although that was yet to be determined.
"If you put hay into a shed, there's always an element of risk of it heating up and catching alight. There is always that risk, but you do your best to mitigate that," he said.
-Additional reporting RNZ
-APL
