The Buller River is a river in the South Island of New Zealand. The Buller has the highest flow of any river in the country during floods, though it is only the 13th longest river; it runs for 177 km from Lake Rotoiti through the Buller Gorge and into the Tasman Sea near the town of Westport. A saddle at 710 m separates the Buller from the Motupiko River and th…The Buller River is a river in the South Island of New Zealand. The Buller has the highest flow of any river in the country during floods, though it is only the 13th longest river; it runs for 177 km from Lake Rotoiti through the Buller Gorge and into the Tasman Sea near the town of Westport. A saddle at 710 m separates the Buller from the Motupiko River and that is divided from the Wairau River by a 695 m saddle, both aligned along the Alpine Fault, as is the top of the Buller valley.