But five of the coins were minted in Kilwa and are believed to be about 1,100 to 1,200 years old (from about A.D. 900), ABC reports.
"It's a very fascinating discovery," McIntosh told CNN. "Kilwa coins have only ever been found outside of the Kilwa region on two occasions.
"A single coin was found in ... Zimbabwe, and one coin was found in the Arabian Peninsula, in what is now Oman, but nowhere else," McIntosh said. "And yet, here is this handful of them in northern Australia - this is the astonishing thing."
Will 5 coins rewrite history?
The Eurocentric view of history holds that Australia, populated by Aboriginal settlers for some 60,000 years, was "discovered" by European explorers in 1606.
But since the discovery of the ancient coins, which came to the attention of McIntosh before Isenberg died in 1991, that history may need to be rewritten. McIntosh also has the old map showing where the coins were discovered.
This July, McIntosh will carry that map back to the Wessel Islands, where he's leading an international team of researchers intent on solving the mystery of how the coins found their way to a remote beach in Australia.