A powerful cyclone has slammed into north-eastern Australia, destroying homes, upending trees, cutting power lines and causing flash floods, while a second storm severed communications to a northern island where heavy damage was expected.

Two powerful cyclones have hit northern Australia, knocking out power to thousands, tearing roofs off houses and prompting coastal residents to flee their homes.

Environmentalists have expressed anger after it emerged that two US fighter jets dropped four unarmed bombs into Australia's Great Barrier Reef Marine Park when a training exercise went wrong.