Australia–Indonesia border

The eastern segment of the boundary was established by both the 1971 and 1972 treaties, the former delimiting the border from A3 to A12 and the latter extending it from A12 to A16, which lies on the eastern border of the Australia – East Timor joint petroleum development area. Points A1, A2 and A3 form part of the Indonesia – Papua New Guinea border which was also established under the 1971 treaty when Papua New Guinea was still an Australian territory.

The 1972 treaty brought the boundary to Point A25 to the north-east of Australia's Ashmore Island. The 1997 treaty extended it further west, terminating at A82 in the Indian Ocean where Australia's and Indonesia's exclusive economic zone limits diverge.

Christmas Island is an external territory of Australia located in the Indian Ocean 186 nautical miles (344 km) south of the southern coast of Java. The maritime boundary between the island and Indonesia which was established by the 1997 treaty is a modified median line which lies closer to the island by virtue of it being an isolated island lying next to the coastline of a larger country.[5] The boundary generally runs east-west and terminates both ends where the boundary of the Australian exclusive economic zone created around Christmas Island diverges from the Indonesian EEZ. Unlike the boundary in the Arafura and Timor Seas, there are no separate boundaries for the seabed and water column.

A separate "water column boundary", essentially an exclusive economic zone boundary for the ownership of fishery resources, was created under the 1997 treaty. The boundary starts from the Australia-Indonesia-Papua New Guinea tripoint at 10° 50' 00" S, 139° 12' 00" E, which is Point A3 of the seabed boundary and designated as Point Z1 of the water column boundary. It runs to the south of the Australia-Indonesia seabed boundary and terminates in the Indian Ocean where the Australian and Indonesia EEZ diverge. The boundary essentially follows the Provisional Fisheries Surveillance and Enforcement Line (PFSEL), a non-treaty status agreement made in 1981 between the two countries, and is generally based on the median line principle measures from the Australian mainland and Indonesian archipelagic baseline.

The 1997 treaty essentially separates the sovereignty of the seabed and water column in the area between the water column boundary and the seabed boundary, whereby Australia has sovereignty over the seabed and its resources, while Indonesia has sovereignty over the water column and the resources which live in it.

Although the 1997 treaty did not "close" the Timor Gap by establishing the seabed boundary in the area, it however established the water column boundary, which virtually corresponded with the boundary between Zone A and Zone B under the Timor Gap Treaty. With the independence of East Timor, this stretch of the water column boundary between Point Z28 and Point Z36 was no longer applicable. The stretch is now replaced with a "water column jurisdiction" line drawn by the Treaty on Certain Maritime Arrangements in the Timor Sea between Australia and East Timor signed on 12 January 2006 which corresponds with the southern border of the Australia-East Timor Joint Petroleum Development Area (JPDA), with Australia having control over the water column south of the line, and East Timor to the north including over the entire JPDA.

The first two treaties to establish the seabed boundary between Australia and Indonesia utilised the "natural prolongation of the continental shelf" principle. Especially for the second treaty, Australia argued that the deep Timor Trough was the natural edge of its continental shelf. The resulting boundary was therefore located closer to Indonesia and had potentially denied considerable share of petroleum resources from the seabed.

Despite the principle being agreed to via negotiations, its use had been a constant point of discontent on the part of the Indonesians, with its former Foreign Minister Mochtar Kusumaatmadja saying that the country had been "taken to the cleaners" by Australia.[9] With the 1982 United Nations Convention on the Law of the Sea, Indonesia argued that the median line – the equidistant line from the baselines of both countries – was the correct location of the boundary. The growing strength of Indonesia's argument over Australia's natural prolongation principle had a significant bearing on the subsequent treaties between the two countries, namely the Timor Gap Treaty of 1989 and the 1997 Australia–Indonesia Maritime Delimitation Treaty.

The differing principles on where the boundary was to be located resulted in the inability to reach agreement to close the Timor Gap with a permanent maritime boundary. A resources sharing joint development area arrangement instead had to be agreed to. Further details on this in the following section.

The 1997 treaty saw Australia's natural prolongation argument prevail as far as the seabed (essentially the continental shelf) claim is concerned. However, Indonesia's median line argument was accepted for establishing its rights over the water column or exclusive economic zone, resulting in the separation of the two types of boundaries. The treaty recognises Indonesia's full ownership of living resources in the water north of the water column boundary and south of the seabed boundary. The water column boundary, essentially the median line, has in fact been a de facto EEZ boundary since such a line was established by the Provisional Fisheries Surveillance and Enforcement Line (PFSEL) which was agreed to in 1981.[5]

Australia and Indonesia were engaged in negotiations to solve a dispute over overlapping claims to the continental shelf in the Timor Gap when East Timor was annexed and became part of Indonesian territory in 1975. After the two earlier seabed boundary treaties, Indonesia was determined that the seabed boundary that would close the Timor Gap would lie along the median line between the two countries. Australia however saw that its continental shelf should stretch to the Timor Trough, a deep trench in the Timor Sea which was located 40 nautical miles (74 km) from the southern shore of Timor and at least 250 nautical miles (460 km) from Australia.

Talks began in 1979 but there was no agreement over the permanent maritime boundary. Instead, the two countries settled for a "sovereignty neutral" agreement in 1989 which allowed joint exploitation and equal sharing of petroleum resources in a "zone of cooperation" which lay in between the extremes of the claims by the two countries.

The 1989 treaty was no longer valid once East Timor seceded from Indonesia in 1999, and the new state was not bound by the treaty. Indonesia had also informed Australia later that year that it was no longer bound by the Timor Gap Treaty.

The dispute over the Timor Gap now is between Australia and East Timor and does not involve Indonesia. However, Indonesia may have to become a party in negotiations to establish the Australia-East Timor-Indonesia border tripoints if East Timor chooses not to be bound by the current "width" of the Timor Gap decided by Australia and Indonesia (through the 1972 treaty) without consultation with the East Timorese, or with Portugal, the colonial administrator of East Timor at the time.