Three Australian soldiers were killed and two wounded in Afghanistan after a man in an Afghan army uniform opened fire and another two Australian soldiers were killed in an unrelated helicopter crash.
The nation's prime minister said the deaths made it Australia's worst day in the ongoing war.
The first three soldiers were killed while relaxing at a base late yesterday, when a man wearing an Afghan National Army uniform opened fire on them with an automatic weapon at close range, Air Marshal Mark Binskin, vice chief of the Australian Defence Force, said.
Australian soldiers at the base in Uruzgan province returned fire, but the shooter scaled a fence and escaped from the base, Mr Binskin said.
The Australians tried to revive their comrades, but the wounds proved fatal. One of the two wounded soldiers sustained a serious gunshot wound and was evacuated to another base for further treatment. He was in satisfactory condition. The second wounded soldier was treated at the scene.
Australian and Afghan soldiers were still hunting for the killer today, Mr Binskin said. He would not release further details of the attack, and would not speculate on the gunman's motive.
In a second, unrelated incident early today, two Australian soldiers died after their helicopter rolled over while landing in Helmand province. A crew member on board was also wounded.
"Australia's lost five men today. They were soldiers who were committed to serving our nation," Mr Binskin said. "I cannot begin to describe the overwhelming grief their families are experiencing. This is a terrible day for all of us."
Prime minister Julia Gillard, in the Cook Islands for a forum of Pacific nation leaders, said she would be returning to Australia early in the wake of the deaths.
"In a war of so many losses, this is our single worst day in Afghanistan," she said. "Indeed, I believe this is the most losses in combat since the days of the Vietnam War and the Battle of Long Tan. This is news so truly shocking that it's going to feel for many Australians like a physical blow."
So-called "green-on-blue" attacks - in which Afghan colleagues or militants disguised as them turn on international forces - have been on the rise in recent years. Last year, four Australian soldiers were killed by Afghan troops.
Thirty-eight Australian soldiers have been killed in the more-than-decade-long Afghanistan war. Australia has 1,550 troops in Afghanistan and makes the largest military contribution of any country outside Nato.