About 150 people will attend the event, which will start at 11am at the Mornington Tavern.
One of the organisers, Dave Ross, was delighted with the response, saying it was some time since a get-together had been held and people thought it was "a great idea".
Stock and station business Donald Reid and Co Ltd was established in Dunedin in 1878, while the Otago Farmers Co-operative Association of New Zealand Ltd was established in 1895. Reid Farmers was formed in 1974.
Directors, as well as staff covering the "whole spectrum" of the company, had been invited. The oldest of those attending were in their late 80s.
Memorabilia would be on display, including many old photographs, and a DVD of the last days of the Burnside stock sale would be played.
Brought up on a dairy farm, Mr Ross joined the Otago Farmers Co-op in about 1969, as a stock agent servicing the Otago Peninsula and Port Chalmers area, having previously sold tractors and farm machinery.
Describing it as an "A1" company to work for, he stayed for about 35 years and said he would "do it again".
He particularly enjoyed the camaraderie among all the clients.
Not completely retired, Mr Ross has a lifestyle block and runs a few sheep.
He was looking forward to catching up with old colleagues, some of whom he had not seen for years.