Australian police ended a 16-hour standoff today when they rushed a downtown coffee shop where a man had been holding 17 people.
The clash left two hostages and the gunman dead and four others wounded.
The prime minister offered condolences to the family and friends of the hostages who were killed.
He also wished a speedy recovery for the injured.
He says Canada mourns with Australia.
"We are also outraged by this barbaric act of depravity on innocent civilians who were merely going about their daily routine," he said in a statement.
The gunman was identified as Man Haron Monis, an Iranian-born, self-described cleric who once was prosecuted for sending offensive letters to families of Australian troops killed in Afghanistan.
Monis was also facing charges in the death of his wife last spring and in a sexual assault case.
Harper, who has cultivated a reputation for taking a hard line against terrorism, didn't directly label the incident a terror attack, but did allude to the need for international action.
"Canada and our allies will stand firm and will stand together against those who would threaten peace, freedom and democracy and all of the values we so cherish."