Even after Captain James Cook (1728-1779) sailed through the Endeavor Strait between Cape York, the northernmost point of the Australian mainland, and Thursday Island to the north in 1770 (south of the Torres Strait), he didn’t claim to have proved that New Guinea was an island separate from what was later known as Australia. He cautiously wrote that “as I believe it was known before, but not publicly, I claim no other merit than the clearing up of a doubtful point.”
     Pedro Fernandez de Quirós (ca. 1565-1615) had claimed in 1606 that New Guinea and Australia were one continent. But a document written by Jean Luis Arias, a lawyer in Chili between 1614 and 1621 refers to New Guinea as “a Country encompassed with water" which shows that Spain knew better.
     Willem Janszoon (ca. 1570-aft. 1629) sailed through what later was known as the Torres Strait in the Duyfken (“Little Dove") between the 18th November, 1605, and June 1606, and later in 1606 Luis Váez de Torres (d. 1613?) also sailed through the strait. The explorer and cartographer, Manuoel Godhino de Erédia (1563-1623) prepared a map to show the Torres' course in 1606. His map is shown on a stamp issued by Papua New Guinea in 1970.

     Luis Váez de Torres was second in command of Quirós’ expedition. He coasted southern New Guinea (called Magna Margarita), sighting Long Reef off Cape York (called Volcan Quemado). He then crossed the Strait that bears his name between the 1st and 9th October 1606. For many years it was assumed that Torres took a route along the New Guinea coast, but in 1980 the Queensland historian and seaman Brett Hilder demonstrated that it was much more likely that Torres took a southerly route, from which he would certainly have seen Cape York, the northernmost extremity of Australia.    
    In 1762 a transcription of the report by Luis Váez de Torres was sent to Alexander Dalrymple (1737-1808) by Juan Bautista Muñoz. Dalrymple had it translated into English and it was printed by Rear–Admiral James Burney in A Chronological History of Discoveries in the Pacific (1806). According to the London Gazette August 19, 1768, “It is no secret that the noted hydrographer, Alexander Dalrymple, who was originally the Royal Society's choice for Commander of the Endeavour, before the Lords of the Admiralty insisted on Lt Cook, has given to Mr. Joseph Banks a secret document he discovered while on expedition in Madras. This is believed to contain the statement of Capt. Luis Váez de Torres that he sailed between two great land masses in the far South more than one Century and a half since.”
     In 1981 the Solomon Islands issued a mini-sheet of four stamps which included an extract of a chart by the Flemish cartographer Cornelius Wytfliet (fl. 1597), Chica sive Patagonia et Australis Terra, which shows that the separation of New Guinea and Australia was already known more than ten years before the voyages of Janszoon, Quirós, and de Torres. He wrote in his Descriptionis Ptolemaicæ augmentum, sive Occidentis notitia brevi commentaro illustrata, studio et opera Cornelli Wytfliet Lovaniensis, 1597, "The Australis Terra is the most southern of all lands. It is separated from New Guinea by a narrow strait. Its shores are hitherto but little known, since, after one voyage and another, that route has been deserted, and seldom is the country visited, unless when sailors are driven there by storms. The Australis Terra begins at two or three degrees from the equator, and is maintained by some to be of so great an extent that, if it were thoroughly explored, it would be regarded as a fifth part of the world."

      Aldama Ayala, Compendio Geografico Estadistico de Portugal y sus posesiones ultramarines, 1855, claimed that Australia was discovered by Fernão Magalhaens in 1520. He based his claim on a map by Fernão Vas Dourado in 1570. It would have been claims like this that formed the basis for Wytfliet's description. More recently Ayala's claim has proved to be a mistake of the northern coast of New Guinea for part of the coast of New Holland,

R.H. Major, Early Voyages to Terra Australis, now called Australia.
London, 1859, pp. xxi-xxiii.

Thanks to Michael YAO for his observations and suggestions.

      The documents mentioned above by Jean Luis Arias and Luis Váes de Torres can be found at the following location: http://gutenberg.net.au/ebooks06/0600361.txt.