The region occupies the southern tip of the North Island, bounded to the west, south, and east by water. To the west lies the Tasman Sea and to the east the Pacific Ocean. At the southern end of the island these two bodies of water are joined by the narrow and turbulent Cook Strait, which is only some 28 km wide at its narrowest point, between Cape Terawhiti and Perano Head in the Marlborough Sounds.

The region covers 7,860 km², and extends north to Otaki in the west and almost to Eketahuna in the east. Physically and topologically the region has four basic areas running roughly parallel to each other along a northeast-southwest axis.

The first of these four regions is a narrow coastal strip of plains running north from Paekakariki. This area, known as the Kapiti coast, contains numerous small towns, many of which gain at least a proportion of their wealth from tourism, largely due to their fine beaches.

Inland from this is rough hill country, formed along the same major geologic fault responsible for the Southern Alps in the South Island. Though nowhere near as mountainous as these, the Rimutaka and Tararua Ranges are still hard country and support only small populations, although it is in small coastal valleys and plains at the southern end of these ranges that the cities of Wellington and the Hutt Valley are located.

The third topological stripe of the region is the undulating hill country of the Wairarapa around the Ruamahanga River. This area, which beomes lower and flatter in the south (terminating in the wetlands around Lake Wairarapa contains much rich farmland. The final section of the region's topology is another section of rough hill country, lower than the Tararuas but far less economic than the land around the Ruamahanga River. Both of the hillier striations of the region are still largely forested.