Sanford is planning to buy Pacifica Seafoods Greenshell mussel and Pacific Oyster businesses for $85 million.
Under the deal, which has received regulatory approval, Sanford will own more than 70 marine farms, water space and mussel longlines, as well as lease, share and contract farming operations.
It will also take a 40% stake in a mussel farm development opportunity in Pegasus Bay, near Christchurch, a mussel and salmon processing facility in Christchurch, 10 marine farm servicing and harvesting vessels, and Pacifica's interest in a co-operative marketing company which markets mussels in China.
Sanford will fund the acquisition using bank loans.
It says buying buying the mussel processing plant in Christchurch is good news for the hundreds of people who work there.
At peak times the plant employs nearly 300 people.
Sanfords managing director Eric Barratt says all workers will be offered jobs on at least the same terms and conditions as they are employed on now.
But he says some restructuring will be needed between Sanford's existing employees and new Pacifica employees where roles may be duplicated
Mr Barratt says Sanfords may upgrade the Christchurch plant in a year or two.
