Cullinane said it had been an incredible journey that started in his home.
"To now see the brand mature safely in the hands of investors who are farmers of such integrity and quality is a fantastic conclusion."
Cullinane will step down from his roles at the company as part of the deal while general manager Nicola O'Rourke will continue to manage the day-to-day running of the business.
Prem Mann, Southern Pastures executive chairman and Lewis Road board member, said Peter had created an amazing brand.
"We're proud to take on the responsibility of building on it to produce the best premium dairy products New Zealand has to offer."
Southern Pastures' farms produce milk under a stringent independently certified 10 Star Certified Values Program which stipulates grass-fed, free-range, climate-change mitigation, human welfare, animal welfare, GMO-free, palm products free, growth hormones free, antibiotic stewardship, and environmental sustainability requirements.
Premium milk produced under the Southern Pastures 10 Star programme is currently used in Lewis Road Creamery's grass-fed butter.
Maan said Southern Pastures earlier this year also took full ownership of NZ Grass Fed Milk Products LP - which was previously a joint venture with Westland Milk Products.
"We're convinced that farming can be a powerful tool for environmental good in addition to providing economic returns," Maan said.
"We have a long-term vision to produce carbon-neutral dairy in New Zealand and we see Lewis Road Creamery as part of that grass-to-glass journey."
The board of Southern Pastures, chaired by Maan and including former All Blacks captain Graham Mourie, Phillip Wight, and Taari Nicholas will become the board of Lewis Road.
It will also appoint an advisory board to be chaired by current Lewis Road board director Alison Barrass and will include California based NZ Grass Fed Milk products director, Deborah Rolf.