The AFP news agency quoted a statement in the name of a group called the Islamic Secret Army giving Canberra 24 hours to withdraw its troops from Iraq.
It said two other people described as East Asians had also been abducted.
"In their statement about the alleged Australian hostages, circulated in the Sunni city of Samarra, purported militants said they had ambushed two US military vehicles on a road between Baghdad and Mosul.
They said Australian Prime Minister John Howard had to announce the withdrawal of troops personally.
"We tell the infidels of Australia that they have 24 hours to leave Iraq or the two Australians will be killed without a second chance," the statement was reported as saying.
Australia was a participant in last year's US-led invasion of Iraq and still has 850 troops there.
