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A Portuguese fleet searching for fabled islands of gold came to New Zealand and Australia 250 years before Captain Cook, an author has claimed in a new book.

The tiny fleet of four ships left the Portuguese base of Malacca on a secret mission in 1522, sailing down the east coast of Australia, bumping along the bottom of the continent as far as the Great Australian Bight before returning to their home port by way of the North Island.

The thesis has been put forward in Beyond Capricorn, a book published this week by Australian historian Peter Trickett, a former New Zealand Herald and Listener senior writer.

He argues that the Portuguese kept their discoveries secret because of their rivalry with the Spanish.

"My theory is that the fleet was hit by a severe southerly storm off the south coast of Australia and pushed across the Tasman Sea to the North Island," he said yesterday.

It has long been known that Cook was beaten to Australia by Dutch navigators, who sailed along the coast of Western Australia in the 1600s as they made their way to their colony of Batavia - present-day Jakarta.

But if Trickett's thesis is right, it means that Europeans charted the Great South Land and NZ nearly a century earlier than previously thought.

His startling thesis began eight years ago, in a Canberra bookshop, when he stumbled on a portfolio of maps reproduced from the Vallard Atlas, a priceless collection of charts which represent the world as it was known in the 16th century.

The atlas, now kept in a climate-controlled vault in the Huntington Library in California, was drawn up by French cartographers in 1547, based on purloined Portuguese charts.

Scholars had noticed that one map closely resembles the coastline of Queensland, aside from a point where it suddenly veers away at a right angle for a distance of about 1500km.

After studying the chart, Trickett theorised that the French map-makers had wrongly spliced together two of the Portuguese charts they were copying from.

With the help of a computer expert, he divided the map in two and rotated the lower half by 90 degrees.

The chart fitted almost exactly the east coast of Australia and its south coast as far as Kangaroo Island, off present-day South Australia.

The North Island of New Zealand is also visible on one chart, he claims.

"Once the chart is rotated, you end up with a big island exactly where the North Island is on a modern map. I've checked the distances and they work out."

Trickett says his theory is backed up by the discovery of 16th-century Portuguese maritime artefacts on the coasts of Australia and New Zealand, including a cannonball and helmet "of probable Portuguese origin" found some time in the early 1900s in Wellington Harbour.

He also cites the discovery, in the 1970s, of a lead fishing sinker on Fraser Island, off Queensland. Analysis showed the lead was mined in Portugal or the south of France sometime around 1500.

Trickett believes the Portuguese expedition was led by Cristavao (or Christopher) de Mendonca, who was given secret instructions by King Manuel I to find a fabled land of gold south of Java, alluded to by Marco Polo in the 13th century.

He is convinced that more than 100 place names on the maps match actual geographic features on the coasts of New Zealand and Australia, including Botany Bay - where Cook landed in 1770.

"This may come as a shock for Anglophiles, but the map evidence leaves absolutely no doubt that Portuguese sailed into Botany Bay and charted it 250 years before Cook arrived there in the Endeavour," the author says.

"They were probably there in December 1522. Their chart of Botany Bay was amazingly accurate.

"It is so accurate, in fact, that using this map I was even able to sketch in the main runways of Sydney international airport to the correct scale, without the slightest difficulty."

The Portuguese failed to settle the newly found lands because of an acute shortage of manpower and resources, Trickett believes.

"They were a small country of just two million people and they were increasingly over-stretched because of the huge seaborne empire they had built up."

Had Portuguese military might not been compromised by a disastrous invasion of Morocco in the 1560s, New Zealand and Australia could have been very different countries.

"We'd all be speaking Portuguese, I suppose," Trickett said. "And we'd be a lot better at soccer."

Auckland University history professor Jamie Belich said claims that the French and Chinese discovered New Zealand prior to Abel Tasman in 1642 had also been put forward.

"I think there are a number of theories of this kind and all are highly unlikely."