Carbon dating of the skull found on the banks of a river in northern New South Wales showed it belonged to a Caucasian male and was likely from the 1600s, long before Cook first reached Australia in 1770.
Local police had ordered the test after the intact skull was found about 200 miles north of Sydney. No other skeletal remains were found with the skull.
However, some experts said the finding of the skull does not necessarily mean that another white man beat Captain Cook to Australia's east coast.
"Before we rewrite the history of European settlement we have to consider a number of issues, particularly the circumstances of the discovery," archaeologist Adam Ford told Sydney's Daily Telegraph.
The skull might have been imported into Australia at a later date as part of a private collection, he said.
"The fact the skull is in good condition and found alone could easily point to it coming from a private collection, and skulls were very popular with collectors in the 19th century."
While Dutch explorers arrived in the north and west of Australia in the early 1600s, Cook is believed to be the first white person to have reached the east coast, arriving in April 1770 and claiming the coast for Britain.