One possible - but very much unproven - answer to your question is actually that Europeans did in fact discover that Australia existed before 1606. In terms of Europeans on naval voyages going to Asia rather than the Americas, the Portuguese did indeed get to Asia very early - Vasco De Gama reached India in 1498. By 1510, the Portuguese - unsatisfied with just making copious sums by bypassing the usual overland trading routes between Asia and Europe (i.e., the 'Silk Road') - had constructed forts and trading outposts and then forcibly conquered the Goa area of India. By 1511, the Portuguese had conquered Malacca in modern day Malaysia. Portuguese traders arrived in East Timor - only 400 miles from modern day Darwin - in 1516.

So it would not be surprising at all if the Portuguese had discovered the existence of Australia by the 1520s. Indeed, some have argued exactly this (most famously Kenneth McIntyre in the 1970s). However, the long and the short is that there's no proof. While there are some puzzling bits of evidence related to possible Portuguese discovery of Australia - a contemporaneous map showing 'Java en-Grande' that looks a bit like the Western Australian coastline, some coins found on a beach in the Wessel Islands off the Northern Territory, and the stories of the so-called 'Mahogany Ship' in Victoria - there is no conclusive proof or records of the Portuguese actually discovering Australia. The rest of the map isn't enormously accurate, the 'Mahogany Ship' was quickly lost so who knows what it was, the coins could potentially have gotten to the Wessel Islands via the Makassan trepang trade (people from Makassar in modern-day Sulawesi, Indonesia, went to the North of Australia to hunt trepang - sea cucumbers - from the early 18th century, and perhaps centuries beforehand), etc. Perhaps the Portuguese did discover parts of Australia but the records are lost. It is likely that if the Portuguese discovered Australia, they would have had little interest in what they had seen of it, because Australia did not appear to have spices or other assets that would be worth a lot of money if transported to Europe.

In contrast, what had happened in the lead up to 1606 (as opposed to the 1520s) was that the Portuguese now had some serious European competition who wanted a piece of the action in India and the East Indies. Because of the (Papally-ratified) Treaty of Tordesillas, the Spanish mostly focused on the Americas and the Portuguese mostly focused on Asia (with some exceptions). In the late 16th century, the Dutch successfully rebelled against Spanish rule - not least because of the Dutch had become Protestant, unlike the Catholic Spanish - and were very happy to disregard the Treaty of Tordesillas. By the 1590s, various Dutch companies were formed to send ships to Asia for profit, and in 1602 these companies were amalgamated to form the Dutch East India Company. Broadly speaking, the Dutch East India Company was a very successful venture; by 1606, the Dutch had established rival entrepôts in places like Banten on the island of Java and were attacking Portuguese interests in places like Malacca. Willem Janszoon's 'discovery' of Australia - or at least the first documented landfall by a European - was directed by the Dutch East India Company, and the voyage Janszoon led set out from Banten and arrived in Australia in February 1606. However, Janszoon seemed to be unable to find the potential riches he no doubt hoped to find in Australia. Perhaps a Portuguese navigator almost a century before Janszoon had a similar story - but if they did, we don't appear to have any records of the voyage.