This makes it all the more puzzling that it has chosen to exclude from its showpiece exhibition two important 16th-century French maps of major historical importance which it acquired in the past year and which have an immediate relevance for Australia.

Why the omission - and why is there not even a mention of the existence of these maps in the exhibition's publicity? When pressed, the library offers excuses. But the question must be asked: could the real reason lie in the fact these maps arguably contain conclusive proof it was a Portuguese maritime expedition that first discovered and mapped the coasts of Australia - in 1521-22 - a century before the Dutch on the west coast and 250 years before Captain Cook charted Australia's east coast?

It would seem highly likely that it was someone taking part in this voyage who drew the figure of a kangaroo discovered this week on a 16th century Portuguese manuscript in a New York gallery. Perhaps the image could even represent one of the marsupial inhabitants of Kangaroo Island - which according to Matthew Flinders nearly 300 years later were "so extraordinarily tame" that he and his crewmen could walk up to them and hit them on the head with sticks.

Both of the National Library's recently acquired maps originated in the French Channel port of Dieppe.

The first, drawn by the eminent French cartographer Pierre Desceliers in 1553, depicts the west coast of a southern continent he calls Java la Grande, which is arguably one of the first-ever representations of Australia. Most remarkably of all, on this coastline Desceliers has written a place name that is virtually identical to this location's present-day Australian name.

The place in question is the Abrolhos, off Geraldton. Officially and in the history books, the full name for this group of dangerous reefs and islands is ''Houtman's Abrolhos'' - Houtman being the surname of a Dutch captain, Frederick de Houtman, who sighted them from his ship the Dordrecht in 1619.

The awkward problem here is that abrolhos is a Portuguese word with no connection whatever to the Dutch language. It is actually a compound of two Portuguese words: abrir, a verb meaning ''to open'', and olhos, meaning ''eyes''. So in 16th century Portuguese seamen's slang the word meant ''open your eyes'' - or in the English vernacular, ''keep your eyes peeled'' (for hidden reefs).

The National Library's newly acquired Desceliers map provides the ultimate proof that the Abrolhos was indeed a Portuguese discovery. For at the exact spot on Australia's west coast opposite what we today call the Abrolhos, this historic map clearly displays the name Abrolho (the singular form of ''Abrolhos'') handwritten by Desceliers. And this on a map drawn in 1553 - a full half-century before the Dutch ever appeared on the scene. And if this were not sufficient evidence of authenticity, further north Desceliers' 460-year-old map also depicts unmistakably west Australia's great Ningaloo Reef, the world's largest west coast coral reef. Now World Heritage-listed, the Ningaloo stretches in an unbroken line southward for 250 kilometres from North West Cape, just as depicted by Desceliers. There can be no mistaking it.

The Ningaloo Reef is also depicted in detail on the second 16th century map (or rather collection of maps) that the National Library recently acquired but which, strangely, it also has not put on display in its Mapping our World exhibition. This map is contained in the Vallard Atlas, produced in 1547.

The library's copy of the Vallard is a magnificent, high-quality facsimile edition. Two maps in this atlas depict the west and east coasts of a mysterious southern hemisphere island continent named ''Terra Java'' which has a clear overall resemblance to Desceliers' ''Java la Grand'' but with much more detail.

How can we be sure that these maps have their origins in a Portuguese voyage?

The short answer is that in the Vallard map there are no fewer than 54 descriptive place names on its depiction of the west Australian coast alone (and just as many on a companion map of the east coast) - and nearly all of them are written in Old Portuguese. The few exceptions are in French, for which there is now an unexpected explanation.

As for the voyage that led to these discoveries, its origins are to be found in Portugal in 1519. By then, a mere two decades after Vasco da Gama's epic voyage to India, Portugal under King Manuel I had been transformed from one of Europe's poorest countries to one of its richest, thanks to its monopoly of the spice trade from the East.

But one other lucrative prize still remained to be found - Marco Polo's fabulous ''Island of Gold, the world's largest island'', said to lie ''south-east of Sumatra'' (in present-day terms, in the vicinity of Australia).

At first King Manuel, who had read The Travels of Marco Polo, was sceptical about the Island of Gold. But as promising reports came in from his captains in the East, he became convinced of its reality and in 1519 gave orders for a maritime expedition to be dispatched from India to discover this fabled land.

In that year, an exceptionally large Portuguese armada of 18 big ships sailed for India. King Manuel had decreed that one of his most trusted captains in this fleet, Cristovao de Mendonca, was to be provided with ships to search for the Island of Gold.

Until recently, thanks to Manuel's obsession with secrecy about discoveries, almost nothing had been found in the Portuguese archives about the Island of Gold voyage and researchers had to rely almost entirely on the map evidence.

But now a crucial document has come to light. This is an eyewitness account that records the actual departure of Mendonca's fleet of four ships from the port of Cochin, in southern India on May 4, 1521, at the start of their voyage ''to discover the Island of Gold''.

The evidence indicates they sailed first down Australia's west coast.

As with Desceliers' map, the Vallard map resembling Australia's west coast also clearly depicts the Abrolhos reefs, although here they have been given the name (in Old Portuguese) bassa roqua - meaning ''rocky shoal''.

The name ''Houtman's Abrolhos'' first appears on a Dutch chart drawn by Hessel Gerritsz in 1628. The library displays this and another Gerritsz chart in its Mapping our World exhibition along with the claim they are the first depiction of Australia's west coast.

Asked if this means that the library rejects the significance of the Portuguese place names on its own newly acquired maps dated nearly a century earlier, the library's head of maps, Martin Woods, will only say that this question is ''under consideration''.

So how did it come about that the Dutch captain Frederick de Houtman called the dangerous reefs off Geraldton ''Abrolhos''? The answer is simple - almost certainly because he had already seen a Portuguese chart with ''Abrolho(s)'' marked on it. A few years earlier his older brother Cornelis had been thrown into prison in Portugal as a spy after he was caught trying to obtain secret navigational information, including charts and shipping routes to the Indies. Cornelis secured his release only when Dutch merchants in Amsterdam joined forces to pay a huge fine - in return for a pledge from Cornelis to share with them everything he knew.

Mendonca's fleet appears to have finished its charting of the west coast in early summer. The estuary of Perth's Swan River is clearly shown on both maps. In the Vallard it has been given the name Rio Plata (''Silver River''). A shipping record indicates the ships were at the Portuguese base of Malacca on January 10, 1522, loading provisions for the second stage of their expedition - to map the Island of Gold's east coast.

The National Library's Desceliers map depicts Australia's east coast only as far as the Tropic of Capricorn. The Vallard map, on the other hand, can be shown to depict not only the entire east coast but also a large part of Australia's south coast as far as Kangaroo Island.

The northern and central sections of the this map have an immediate, undeniable resemblance to the coasts of Queensland and northern NSW. Numerous islands, large and small, of the Great Barrier Reef are depicted, and this area has been suitably named Costa Dangeroza: ''Dangerous Coast''.

Australian east coast landmarks unmistakably depicted further south include Fraser Island, (given the name Illa Plata: ''Silver Island''), Moreton and Stradbroke islands, the Brisbane River estuary (Bonno Porto, or ''Good Port''), and Botany Bay, with the Georges River shown winding sinuously inland. And remarkably Batemans Bay is even depicted, with the name Rio Basso - ''Shoal River''.

The Portuguese fleet evidently rounded Wilsons Promontory (named Cabo Formoza, meaning ''Cape Beautiful'') and sailed along the south coast as far as Kangaroo Island, off present-day Adelaide before turning back.

I am by no means alone in believing that the NLA's recently acquired maps contain evidence of a Portuguese discovery of Australia. John Molony, Emeritus Professor of History at the Australian National University, recently made a study of Christian saints' names that appear at eight locations on the Vallard east coast map.

His research shows all of these names to have a clear connection with the Portuguese Catholic church in the 16th century - and even more significantly, with Portuguese India.

On the basis of his findings, Molony concludes there can be no doubt that these place names were the work of Portuguese mariners, and that the Vallard map is based on original Portuguese charts.

So why is the NLA not displaying either of its newly acquired Dieppe maps in its blockbuster exhibition?

The library's stated reason (when asked) for not putting its Desceliers map on display is that this is not the original. Strictly speaking this is true - but then the original no longer exists, having been destroyed by a fire in 1915.

So that this map that the library bought from a private collector is the nearest thing anyone will ever get to the original. It is one of only a few facsimile copies utilising a newly devised reproduction process that were made just before the original went up in flames.

In any case the library's excuse rings hollow when one finds that there are already several items on display in the exhibition that are not originals.

Searching for other reasons that could explain why these important maps are not on display, the fact is that they directly contradict the library's stated claim that the two Dutch charts from 1628 featured in its exhibition represent ''for the first time a recorded depiction of the west coast of Australia''.

And, of course, the library also firmly upholds the accepted view that Captain Cook was the first European to sail along and chart Australia's east coast.

Any admission the Portuguese were the first to discover and map the coasts of Australia would amount to no less than rewriting the first chapter of Australia's European history. It would be a bold step for the library to take.

As things now stand, any visitors to the NLA's exhibition who want to see the Desceliers or Vallard maps will have to descend to the basement and ask at the maps room for these items to be brought out of storage.

Mapping the World is an impressive exhibition, but it can be argued that by excluding important evidence in its possession the library is choosing to ignore uncomfortable facts and thereby undermining its claim that the exhibition presents a complete history of the mapping of the Australian continent.

Not everyone will necessarily accept that the Portuguese came first. But it seems regrettable that the library, a respected national institution, has chosen not to put its two most recent early map acquisitions on display so that visitors can assess the evidence and decide for themselves.

Peter Trickett is a former journalist and editor who specialised in science and history. His book Beyond Capricorn, describing the Portuguese discovery of Australia, was published in 2007.