Police said the man, thought to be Japanese, died at popular tourist spot Shelly Beach near Ballina, some 187 kilometres (116 miles) south of Brisbane.
A group of surfers helped get him out of the water and first aid was administered, but to no avail.
“We have a young male we think is a Japanese national, but enquiries are still ongoing. Around 9.45 this morning he has been the victim of a shark attack,” Detective Inspector Cameron Lindsay told the local Northern Star newspaper.
“There was quite a number of witnesses we’ve been talking to. We believe that either the attack, or just prior to the attack, was captured on video.”
A local cafe owner, identified only as Karen, told reporters the shark came from nowhere.
“It just came up between a bunch of surfers,” she said. “They weren’t even that far out. He lost both his legs.”
Police only described his injuries as “substantial”.
“At this initial stage we believe he died because of lack of blood despite some really heroic efforts by the people that have tried to save him,” Lindsay said.
Beaches in the area were closed after the fatality, which came just one day after Jabez Reitman, 35, was attacked while sitting on his surf board off Seven Mile Beach near Byron Bay, only 30 kilometres north of Ballina.
He suffered a cut to his back and puncture wounds to his buttocks.
“Something just jumped out of the water and just grabbed me at my hip and dragged me off my board and took me under the water,” he told the Australian Broadcasting Corporation.
“I just came back up and I paddle, paddle, (and) paddled about 60 metres back into the shore.”
Sharks are a regular feature in Australian waters, and swimmers and surfers have had their share of encounters with the animals this summer
Fatalities, however, are rare — although two teenagers died in shark attacks in December, one on the country’s west coast and one on the east.
Experts say attacks by sharks are increasing as water sports become more popular.