The 33-foot boat had just been refueled when the blast occurred, The Age reported. The couple who died, described as being in their 60s, were standing just above the vessel's fuel tanks.

Garry Patrick, who was in his boat sales office nearby, said the explosion was so concentrated the boat "was gone in one bang." It occurred as the ignition was turned on.

Patrick pulled the couple's son from the water.

"He was just crying that his parents were still in the water somewhere, but we couldn't see, the flames were too intense," Patrick. "There was no sign of them. All that was sitting on the water were bits of wood and debris and fuel burning so hot you could feel the heat on your face."

The son's partner was seriously injured. Her sister and nephew suffered minor injuries.