Origin of the Western Australian Land Boundary

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The internal political boundaries of Australia, according to common belief, were derived from the Treaty of Tordesillas in 1494, which divided the New World between Spain and Portugal along a western meridian in the Atlantic. Later, when the lucrative Spice Islands in the Moluccas of Indonesia were discovered by the Portuguese sailing east in 1512 and the Spanish Magellan expedition sailing west in 1521, the Treaty of Zaragoza in 1529 attempted to clarify the anti-meridian of the eastern hemisphere to designate who could rightly claim possession. Because the Moluccas are close to the anti-meridian it was inevitable that both Spain and Portugal would vigorously defend their rights to occupation under the terms of the treaty such that to this day the anti-meridian has never been fully agreed upon. Within the Australian context, and supposedly based on the Treaty of Tordesillas, the three anti-meridians of 135°E, 129°E and 141°E in that order have been used to designate Australia’s internal political boundaries, but uncertainty remains on the origin of each. Historical maps and documents from the 16th and 17th centuries clarify the credibility of the various anti-meridians and show that the only valid contender is the Portuguese anti-meridian of 135°E. This line was used by Tasman in 1644 to divide the Australian continent between Hollandia Nova and Terra Australis, and later by Governor Darling in 1785 to designate the boundary between New Holland and New South Wales. The derivation of the Western Australian border at 129°E appears to have been a convenient choice to protect New South Wales settlements at Port Essington and Melville Island, west of the 135°E Portuguese anti-meridian, rather than to any speculative alternative Portuguese anti-meridian derived from the Treaty of Tordesillas.