The man was rushed to shore after being bitten near Kennedy Shoal, a shallow reef around 45 km (28 miles) off the coast of the state of Queensland in northeast Australia, a Queensland Police Service spokesperson said.
The man was met by an ambulance but died shortly afterwards, the spokesperson said, without identifying him.
The death follows that of a 38-year-old bitten off an island near Perth in Western Australia on May 16.
There are around 20 shark attacks in Australia every year but the vast majority are not fatal, according to data from conservation groups. Far more people drown on the country's beaches.
Dozens of beaches along Australia's east coast, including in Sydney, were closed in January after four shark attacks in two days. Those attacks came as heavy rains created murky water that attracts sharks and reduces their visibility.
