The Australians were killed in two separate actions just hours apart late Wednesday and early yesterday.
The first took place at a base in Uruzgan province, when a man in an Afghan army uniform opened fire on Australian soldiers, killing three and wounding two, according to Air Marshal Mark Binskin, vice chief of the Australian Defense Force.
Hours later, two Australian soldiers died and a crew member was injured when their helicopter rolled over while landing in Helmand province.
"In a war of so many losses, this is our single worst day in Afghanistan," Prime Minister Julia Gillard 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."
Eighteen Australian soldiers were killed in the Battle of Long Tan in 1966. Tom Vasey, a spokesman for the Australian War Memorial, said five Australians were killed in a 14-hour period in 1971 during the Battle of Nui Le, making that the last time so many died so quickly in a combat zone.
Gillard said the latest bloodshed would not speed up Australia's plans to withdraw troops only after completing their main mission of training an Afghan battalion for securing the restive Uruzgan region. That withdrawal could happen as soon as early next year.