Pullin, Australia's flagbearer at the 2014 Winter Olympics in Sochi, was found by a snorkeller on an artificial reef just before 11:00 local time (0100 GMT) in waters off Palm Beach, state broadcaster ABC reported.
Lifeguards and paramedics attempted to resuscitate the 32-year-old on the beach after pulling him from the water. Queensland Ambulance Service officer Justin Payne said he did not survive.
Another diver had found Pullin on the sea floor in "very upsetting" circumstances, said Gold Coast Police District Duty Officer Chris Tritton.
"He didn't have an oxygen mask, we understand he was free diving and spearfishing out on the reef," Tritton told Australian broadcaster Channel Nine.
"It appears he was diving alone. There were other divers out there but he was not with a friend."
Brisbane-based newspaper the Courier-Mail earlier reported that Pullin was thought to have "suffered a shallow water blackout".
Pullin, nicknamed "Chumpy", won world championship titles in snowboard cross in 2011 and 2013, competed at the 2010 Winter Games in Vancouver and placed sixth in his event at the Pyeongchang Games in South Korea two years ago.
"It is an incredibly sad day," Geoff Lipshut, the chief executive of the Olympic Winter Institute of Australia, told Reuters.
Lipshut said Pullin had retired from his sport last month but had yet to announce it.
"Alex 'Chumpy' Pullin was one of our great winter sports pioneers ... He attacked every day with intensity and purpose," he said.
The New South Wales Institute of Sport wrote on Twitter: "Alex ‘Chumpy’ Pullin was an extraordinary individual who pursued his passions in sport and in life. This loss is tragic to everyone he inspired and loved."