The plains can be divided into two broad areas. These two areas are separated by the uplands of the Hokonui Hills, which lie to the west of Gore.
The northwestern area lies to the north of the city of Invercargill, and is formed by several large rivers, the Aparima, the Oreti, and the Makarewa. They stretch inland for over 45 kilometres from the coast of the Foveaux Strait, and cover an area of close to 1500 km².
To the northeast of Invercargill, the plains of the Mataura River and its tributary the Waimea River stretch inland as far as the town of Gore. At their southern extreme they become the Awarua Plain, a large area of marshy land which stretches from near Bluff to the foot of the Catlins in the far east. Between them, the lowlands around the Awarua Plain and Mataura River cover a further 2000 km².
A further small area to the west, around the mouth of the Waiau River, can calso be considered part of the plains.
The Southland Plains include some of New Zealand's most fertile farmland.