The biplane disappeared Monday on its way to Caboolture, north of Brisbane, soon after the pilot radioed he was in trouble, the Australian Broadcasting Corp. reported.
Australian Maritime Safety Authority spokesman Mike Barton said the plane must have "hit the ground exceedingly hard" when it crashed about 8 1/2 miles north of Borumba Dam.
"The plane is not in a condition that you would recognize as a plane," Barton said. "So they have hit the ground exceedingly hard and the aircraft is fundamentally destroyed."
He said it was likely the six people were killed on impact.
The 1934 red De Havilland Dragon DH, one of four in the world, was returning from an air show at Monto, ABC said.
"I personally knew the pilot and I think the antique airplane community are going to be quite upset for a while that they've lost him," Barton said.
Search crews had an idea about the plane's location because of a cellphone on board.
Barton said an inspection into what caused the crash was under way.