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The barn is part of a group of buildings, including a homestead and outbuildings, known as Butler's Farm, on State Highway 8, which have been given category 1 status by the New Zealand Historic Places Trust.
The farm was established in the early 1870s.
Property owner Nick Taylor, a pilot who moved from Wellington to oversee and work on the project, said the wall straightening was significant as now he would apply for resource and building consents to undertake further work.
He planned to turn the barn into some sort of rental accommodation.
The barn was the only building of the group that work had started on so far.
"I'm going to do them one at a time," Mr Taylor said.
"Each building will be done really well. I'm in no rush. It's going to take years."
He declined to reveal the cost of the project.
The side walls of the barn had been put under increased pressure by a roof that had been added between 1970 and 1990, according to a conservation plan report commissioned by Mr Taylor and prepared by Queenstown architects Jackie Gillies and Associates.
A failure to tie the roofing structure into a wall plate ... has led to the partial collapse of the west wall, the report said.
As the west wall collapsed, it pulled the east wall out of alignment, Mr Taylor said.
Pushing the west wall back into line also straightened its eastern counterpart.
St Bathans stonemason Keith Hinds was restoring the walls of the barn using the original schist stone that had collapsed and was still on the property.
Mr Hinds had attempted to prop up the west wall but, due to its weight, Breen Construction had to be called on, as it had big enough props to straighten the wall.
Once the props were in place they were wound up manually, lifting the wall into line.
Breen project manager Saul Bedford said his company had been involved in a similar project at Old Cromwell Town.
Butler's Farm is named after John Butler, who bought it in 1878 and later moved into orcharding.
The farm was also used for dairying during the early days of the industry, before trees were planted in 1915.