St Aloüarn made the first formal European claim of sovereignty — on behalf of France — over the west coast of Australia,[1] which was known at the time as "New Holland".

Much of this west coast had already been charted by other mariners from the Netherlands, following a landing by Dirk Hartog in 1616, who left a commemorative plaque recording his visit.

[5] In 1771, shortly after the death of his wife,[6] St Aloüarn was approached by a colleague, Yves de Kerguelen, who asked him join an expedition to New Holland.

[4] On 30 April 1771, they left Port Louis in two small vessels: Kerguelen on board the 24-gun fluyt Fortune and St Aloüarn commanding the 16-gun storeship Gros Ventre.

[6] After also landing a party on the island, St Aloüarn continued towards Australia and a rendezvous point at Cape Leeuwin, arranged earlier with Kerguelen.

[10][11] Members of Mengaud's ceremonial team raised the white ensign on the island and buried a bottle containing a document stating what had occurred, alongside two silver écu coins, worth six Livres tournois (Francs).

[12] This occurred in sight of Cape Inscription, where in 1696 the Dutch mariner Willem de Vlamingh had also left a commemorative plate recording his visit and that of Dirk Hartog in 1616.

[8] Gros Ventre then visited Batavia (Jakarta) in the Dutch East Indies, where St Aloüarn and some of his crew contracted "tropical diseases".

During the late 20th century, historian Leslie Marchant, one of whose specialities was the French exploration of Australia,[15] and others, led expeditions that attempted to find the site of St Aloüarn's proclamation.

[16] Searches continued for a bottle reportedly buried by St Aloüarn's crew, containing a document proclaiming France's annexation of Western Australia.

There is anecdotal evidence that the proclamation was found decades earlier by a stock worker, was kept at the homestead of a sheep station operating on Dirk Hartog Island at the time and was later destroyed by fire.