An experienced advertising source familiar with the business told the Herald the pitch came down to a tussle between FCB and Clemenger Group (led by 99).
He said that he had never heard of an agency losing an account at the due diligence stage and that was normally just a final formality.
The source said a full retail account of this magnitude could be worth around $5 million in revenue to a creative agency.
Both these agencies know the Farmers account well, having worked on the business in recent years.
FCB developed Farmers' Christmas campaign in 2017, while 99 previously served as the company's retail and creative agency.
More recently, Farmers has been taking most of its advertising work in-house, establishing a large creative department made up of photographers, Mac operators, digital marketing specialists and more.
The return to an agency of record marks a strategic shift under Dunn, who last year took over as the marketing boss from Dean Cook, who had been with the business for 21 years over two stints.
The source said moving back to an outsourcing model - even if only for part of the business - alters the skill sets required within the Farmers creative department and could possibly lead to staff changes.
Dunn also wouldn't comment at this stage on what the change could mean for the in-house creative team.