The vendor has been disclosed as the Theland Purata dairy farming company, owned by investors in China.
Synlait is also 65% overseas owned, the majority owner being Chinese listed company Bright Dairy and Food.
The OIO said the land was originally owned by Synlait Farming and was sold to Theland Purata in 2016.
It is now being bought back by Synlait Milk for $25.7 million to safeguard water access from bores and for discharge irrigation.
Synlait also intends to build a rail siding and shift 16,000 truck movements annually from the roads.
OIO approval has also been granted for the purchase of Dairyworks brands including Rolling Meadow and Alpine and its Canterbury premises with 240 staff.
Dairyworks, bought for $112m, specialises in processing, packaging and marketing dairy products including cheese, butter, ice cream and milk powder.
