The Sounds-Wellington Eco-region is defined by two distinct areas. The Wellington district features steep, faulted hills, two harbours, the Pāuatahanui estuary, and Hutt River valley, all enduring strong, prevailing west to northwest winds and frequent gales. The Cook Strait district, defined by its maritime climate, comprises highly exposed, steep coastal escarpments, terraces, headlands, and islands along the strait, also experiencing frequent gales. Though many species span both sides, the strait itself is a notable biogeographic barrier.