The two AV-8B Harrier aircraft were forced to release the bombs because they were low on fuel and could not land with the amount of ordnance on board, the Navy said in a statement.
"The Harriers had intended to drop the ordnance in Townshend Island Range but controllers reported the area was not clear of hazards," the statement said. "Due to low fuel and inability to land with the amount of ordnance they were carrying, the on-scene commander determined it was necessary to designate an emergency jettison area for the ordnance."
Two of the bombs were explosive and were disarmed before they were dropped, CNN reported. The other two were non-explosive bombs.
After the incident, the Australian military said the bombs would not likely harm anyone or marine life, The Australian reported.
The bombs posed "minimal risk or threat to the public, the marine environment or civilian shipping transiting the reef area," a spokesman said. "The incident is being investigated by the United States and findings will be provided to the Great Barrier Reef Marine Park Authority so that the way ahead can be mutually agreed."