Two US fighter jets dropped four unarmed bombs in Australia's Great Barrier Reef Marine Park when a training exercise went wrong, it has emerged.

The two AV-8B Harrier jets launched from aircraft carrier USS Bonhomme Richard each jettisoned an inert bomb and an unarmed explosive bomb in the World Heritage-listed marine park off the coast of Queensland state on Tuesday, the US 7th Fleet said.

The four bombs were dropped in more than 50 metres (164ft) of water away from coral to minimise 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 Sabre, which brings together 28,000 US and Australian military personnel over three weeks.

The US Navy and Marine Corps were working with Australian authorities to investigate the incident, the navy said.

Graeme Dunstan, among environmentalists and anti-war activists demonstrating against the joint exercise, said the mishap proved that the US military could not be trusted to protect the environment.

"How can they protect the environment and bomb the reef at the same time? Get real," he said from the Queensland coastal town of Yepoon near where the area of the war games.

The Great Barrier Reef is the world's largest network of coral structures rich in marine life that stretches more than 1,800 miles along the Australian north-east coast.