Home Affairs Minister Peter Dutton says the community must step up to help authorities prevent terror attacks even though the man behind the fatal strike on Melbourne's Bourke Street last Friday was on their radar.
He says more than 400 people are currently being investigated by police and intelligence services but need a tip-off or alert from the public to stop a spontaneous act.
"It is important for us to get as much information from the imams, from spouses, family members, community members, council workers, people that might be interacting with those that might have changed their behaviours, that they think have been radicalised," he told reporters in Brisbane on Sunday.
"The idea that community leaders would have information but withhold it from police or intelligence agencies is unacceptable and we're best to be honest about the problem that we've got so that we can address it."
Authorities were already aware of the man behind the death of well-known Italian restaurateur Sisto Malaspina, 74, and the stabbing of two other men including Tasmanian businessman Rod Patterson, who remains in the Alfred hospital.
Hassan Khalif Shire Ali, 30, had previously been spoken to by ASIO officials and his passport was cancelled in 2015 over concerns he would travel to Syria.
He turned his attention to home soil on Friday when he parked a four-wheel drive laden with gas cylinders on Bourke St and stabbed three men, killing one.
Mr Dutton says the government's community engagement programs, including those run by his department, are sowing positive relationships that have led to effective intelligence gathering.
But he also says a tip-off could have backed up an apparent gap in intelligence and potentially stopped what he has described as an unsophisticated attack.
"Where there is not (that information), where someone makes a spur-of-the-moment decision - under the influence of drugs or alcohol - the police can't contemplate every circumstance," he said.
"These are complex matters and we need the evidence to supplement the intelligence we have."
Mr Dutton said technology, including encrypted apps, had made it harder for authorities to tap phone calls or keep tabs on people of interest.
"There is a real black spot for us and that is a vulnerability," he said.
Mr Dutton said authorities did not believe Shire Ali was a member of Islamic State and was not known as being an imminent risk.
"There was no evidence available to the police or I'm advised to ASIO that any attack was imminent or that he had been part of planning," he said.
"In relation to his connections with ISIL or with any terrorist group .... there's not, as I'm advised a membership of an organisation or a definite link to ISIL."
He said discussions around what could be gleaned from Friday's incident along with attacks overseas to prevent similar incidents in future were underway.