So far, so vague. But with the dramatic events unfolding only a day after Isis urged its followers to attack Westerners, including Australians, and less than a week after mass raids in Sydney and Brisbane foiled a beheading plot, according to police, public fears of a domestic terrorist attack have ratcheted up yet another notch.
And, at the risk of appearing cynical, that's the perfect environment for repressive new security and counter-terrorism laws to be rushed through Parliament - which is exactly what is happening this week. The new laws, among many other things, expand police powers to arrest and detain suspects, and permit them to secretly seize passports and search properties without advance warning. Reportedly, they will also enable the intelligence agency ASIO to monitor the entire Australian internet with just one warrant.
With an eye, perhaps, to sceptics who have questioned whether Isis really poses a threat to Australia, Abbott has been assiduously stoking fears. "I cannot promise that hideous events will never happen on Australian soil," he declared last week. And yesterday, commenting on the Melbourne incident: "This indicates that there are people in our community who are capable of very extreme acts."
Does it, though? We know that a teenager posted photos of himself on Facebook in military camouflage, along with an abusive message about "the dogs AFP and ASIO who are declaring war on Islam and Muslims". He may have waved an Isis flag in public. He assaulted two police officers with a weapon. Whether that all adds up to something more sinister than youthful folly, and a reckless act of violence, is not clear. And with Haider dead, we'll probably never know.
Never mind, it puts the fear of God, or Allah, or Isis, into the population. And that's good for the 24-hour TV and radio networks, good for the defence and security industries, and, above all, good for the Government. The Coalition's ratings have risen to a five-month high.