The man was attacked at Cull Island near the town of Esperance in Western Australia state, the state's primary industries department said in a statement.
"A man received fatal injuries after being bitten by a reported white shark," the department said.
Surf Life Saving Western Australia said he was believed to be a diver.
British man has foot bitten off, another hurt in Australian shark attack
The fatal attack, the first in Australia this year, is the second in the area in less than three years.
A 17-year-old girl was mauled by a shark in full view of her parents near Wylie Bay in 2017. She lost her leg and was pulled from the water, but died shortly after.
In 2014, a young surfer lost parts of both arms after being attacked by a great white, also off Esperance.
Australia has one of the world's highest incidences of shark attacks, but fatalities remain rare.
There were 27 maulings but no fatalities off Australia last year, according to data from Sydney's Taronga Zoo.
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