The coastal environment of Marlborough is made up of two quite distinct geographic areas - the Marlborough Sounds (Sounds) and the East Marlborough coast.
The Sounds are essentially large drowned river valleys lying between mountain ranges, extending from Cape Soucis in the west to Port Underwood in the east. The East Marlborough coast, which is an open sea coast, extends from Robin Hood Bay in the north to Willawa Point in the south.
Together these areas contain 1814 kilometres of coastline, or approximately eighteen percent of New Zealand's total coastline.
The Council's responsibilities for managing the coastal environment are shared with the Minister of Conservation. The area of shared responsibility is called the "coastal marine area" and this is defined as being the area extending from the mean high water springs mark, seaward to the outer limits of the territorial sea (12 nautical mile limit). This includes the inter-tidal area, the seabed, the coastal water and the airspace above the water.