Australians and environmentalists have reacted furiously to the news that two US fighter jets dropped four unarmed bombs on the Great Barrier Reef Marine Park during war games.

The two 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 on Tuesday when a training exercise went wrong, the US 7th Fleet said in a statement on Saturday.

Have we gone completely mad? Is this how we look after our World Heritage area now? Letting a foreign power drop bombs on it?" Larissa Waters, Australian Senator

Australian Senator, Larissa Waters, the Greens party's spokeswoman on the Great Barrier Reef, described the dumping of bombs in such an environmentally sensitive area as outrageous and said it should not be allowed.

"Have we gone completely mad?" she told Australian Broadcasting Corp. "Is this how we look after our World Heritage area now? Letting a foreign power drop bombs on it?".

None of the bombs exploded when they were dropped into more than 50 metres of water. They were dropped away from the coral reef to minimise damage, the statement said.

The pilots conducted the emergency jettison of the bombs, weighing a total of 1.8 metric tonnes, because they were low on fuel and could not land with their bomb load, the US 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 weeks.

Graeme Dunstan, who is among the environmentalists and anti-war activists demonstrating against the joint exercises, 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," Dunstan said.