The four bombs were dropped in more than 50 meters (164 feet) of water away from coral to minimize possible damage to the reef, the statement said. None exploded.
The jets from the 31st Marine Expeditionary Unit had intended to drop the ordnances on the Townshend Island bombing range but aborted the mission when controllers reported the area was not clear of hazards.
The pilots conducted the emergency jettison because they were low on fuel and could not land with their bomb load, the navy said.
The emergency happened on the second day of the biennial joint training exercise Talisman Saber, which brings together 28,000 US and Australian military personnel over three week.
The US Navy and Marine Corps were working with Australian authorities to investigate the incident, the navy said.