Media reports of the Royal Australian Navy and Customs and Border Protection Service's recent breaches of Indonesia's territorial waters contain little detail of the actual transgressions, but it is most likely that they result from confusion over the way in which those waters are defined.
Under the Law of the Sea as laid down within the 1982 UN Convention, territorial waters are generally taken as extending 12 nautical miles out from land. However, there are provisions within the Convention which make special arrangements for nations which have more complex geography. One of the most important components of the 1982 Convention was its recognition in Part IV of the special nature of archipelagic states. This was an innovation which owed much to Indonesia and the Philippines, who lobbied strongly over many years for the special status of archipelagos.
An archipelago is defined within the Convention as a 'group of islands, including parts of islands, interconnecting waters and other natural features which are so closely interrelated that such islands, waters and other natural features form an intrinsic geographical, economic and political entity, or which historically have been regarded as such.'
Effectively, this means that an archipelago is recognised as one entity and that its boundaries are drawn around the whole, rather than around the individual islands and formations within it. This was central to the original concept, one described within Indonesia in relation to its territory as 'land and water.'
The 1982 Convention sets out the rules by which the 'baselines' can be drawn 'joining the outermost points of the outermost islands and drying reefs'. A key provision is that 'the ratio of the area of the water to the area of the land, including atolls, is between 1 to 1 and 9 to 1.' There are other restrictions, amongst them that the length of baselines should not exceed 100 nautical miles, although up to 3% of the total number may extend up to 125 nautical miles.
Indonesia unilaterally declared its boundaries as encompassing everything inside its outer islands as far back as 1957 and formally legislated to this effect in 1960. It modified the legislation in 1998 and, under its provisions and later regulations, provided the UN with the detailed list of baselines and their coordinates required by the Convention in 2009. The baselines themselves were updated in 1998, 2002 and 2008. There remains the need for some corrections in the vicinity of Timor Leste, while a further update is in progress, requiring work around Sumatra to be completed.
Warships (and other vessels) of other nations do have certain rights when transiting the waters of archipelagic states, but in operations related to border security, these do not apply. The point is that one must consider an archipelagic state as a single entity in relation to its maritime boundaries. It is not a just a matter of looking for the nearest piece of land and working in relation to that.
Map courtesy of the CIA World Factbook.