Three Australian commandos were killed when a helicopter crashed in southern Afghanistan on Monday, in the country's deadliest single incident in the long-running conflict, officials said.
One other NATO soldier was also killed, while three personnel were seriously wounded, armed forces chief Angus Houston and Defence Minister John Faulkner said, stressing that the chopper was not brought down by enemy fire.
"It is with a profound sense of loss and sadness that I inform you that as a result of that incident, three of our soldiers have been killed," said Houston.
The crash, which brings Australia's death toll in Afghanistan to 16, comes two weeks after two soldiers from the country were killed in a bomb blast during a sudden spike in foreign casualties.
Faulkner described it as a "tragic day" for Australia and its armed forces.
"This most dangerous theatre of operations has claimed five Australian lives in just two weeks and it's with very great sadness that we are here today to inform you of these casualties."
Faulkner and Houston pledged to continue the mission to Afghanistan, where Australia has about 1,550 troops in Uruzgan province.
Prime Minister Kevin Rudd, who has called the war "unpopular", sent 450 reinforcements last year but has resisted Washington's calls to provide more.