The term Territorial Sea Baseline (TSB) refers to the line from which the seaward limits of Australia's Maritime Zones are measured. These include the breadth of the territorial sea; the seaward limits of the contiguous zone, the exclusive economic zone and, in some cases, the continental shelf.
The territorial sea baseline may be of various types depending upon the shape of the coastline in any given locality:
The Normal baseline corresponds with the low water line along the coast, including the coasts of islands. Under the Convention, normal baseline can be drawn around low tide elevations which are defined as naturally formed areas of land surrounded by and above water at low tide but submerged at high tide, provided they are wholly or partly within 12 nautical miles of the coast. For Australian purposes, normal baseline corresponds to the level of Lowest Astronomical Tide (LAT).
Straight baselines are a system of straight lines joining specified or discrete points on the low-water line, usually known as straight baseline end points. These may be used in localities where the coastline is deeply indented and cut into, or where there is a fringe of islands along the coast in its immediate vicinity.
Bay or river closing lines are straight lines drawn between the respective low-water marks of the natural entrance points of bays or rivers.
Waters on the landward side of the baseline are internal waters for the purposes of international law.