Australians need to be prepared that a London Bridge-style terror attack can happen here at any time, with authorities powerless to catch every threat.
And while our police and intelligence services are on top of emerging threats, it's impossible to catch every one, according to experts.
The dire warning comes after three men in a van ploughed into pedestrians on London Bridge before going on a stabbing spree in the city's popular Borough Markets yesterday.
Seven people lost their lives and 48 people were injured, including four Australians, before the attackers were shot dead.
It is the second such attack in London involving a car ramming into pedestrians in two months.
Khalid Masood rammed into pedestrians on Westminster Bridge on March 22, killing three people before stabbing a policeman to death. He was later shot and killed.
Former Australian Secret Intelligence Service (ASIS) officer Warren Reed said Islamic State was losing the battle in the Middle East and warned the group was desperately appealing to radicalised Muslims to carry out attacks on home soil.
Reed said cars and knives were easily accessible and the reality was nothing could stop someone who is radicalised and psychotic.
He stressed that he didn't want to make people paranoid but it was crucial that Australians were vigilant and aware that this sort of attack could happen here.
Reed said radicalised Muslims, whether psychotic or not, who haven't been willing to go to the Middle East and fight, could easily carry out on an attack here in the comfort of their home countries.
"It's a case of ISIS coming to you rather than you coming to it," he said.
"ISIS is losing the fight in Mosul (Iraq) and will lose Raqqa (Syria) so they're now relying on home grown groups to take up the cause.
"We have to be ready for this to happen in Australia, we can't avoid it."
Former Australian Secret Intelligence Service (ASIS) officer Warren Reed said Islamic State was losing the battle in the Middle East and warned the group was desperately appealing to radicalised Muslims to carry out attacks on home soil.
Reed said cars and knives were easily accessible and the reality was nothing could stop someone who is radicalised and psychotic.
He stressed that he didn't want to make people paranoid but it was crucial that Australians were vigilant and aware that this sort of attack could happen here.
Reed said radicalised Muslims, whether psychotic or not, who haven't been willing to go to the Middle East and fight, could easily carry out on an attack here in the comfort of their home countries.
"It's a case of ISIS coming to you rather than you coming to it," he said.
"ISIS is losing the fight in Mosul (Iraq) and will lose Raqqa (Syria) so they're now relying on home grown groups to take up the cause.
"We have to be ready for this to happen in Australia, we can't avoid it."
He said it was up to families and friends of people who noticed suspicious behaviour to trust police and authorities and report it.
"The problem is we can't catch everyone," he said.
Two women hug after bringing flowers to add to tributes laid on the north side of London Bridge. Photo / AP
According to former intelligence analyst Associate Professor Patrick Walsh, Australians could no longer walk around the streets idly without looking over their shoulder.
Dr Walsh, a senior lecturer in Intelligence and Security Studies at Charles Sturt University, said it wasn't possible to live in a police state.
He also said Australians had to be ultra vigilant particularly in crowded areas and must be willing to report or say something regarding suspicious activity.
"We need to have more situational awareness," he told news.com.au
"The days of idling around the street without looking over our shoulders are long gone."
Dr Walsh said the positive thing was the terror threat was directly in the forefront of those working in the AFP and ASIO.
While the installations such as bollards on the streets was one way of keeping people safer we couldn't rely on the government alone to protect us.
He also agreed that it was impossible to catch and monitor everyone especially since some might not even be under the radar of authorities.
Nor was it economically possible to shut every road and bridge in Australia to keep us safe.
"It's up to all Australians, not just Muslim Australians, to stay vigilant," he said.
A small child lays flowers at a corner tribute in the London Bridge area of London. Photo / AP
"But if someone is radicalised quickly in the privacy of their own living space there's not much we can do."
Dr Walsh also said police everywhere will be hyper vigilant and will now stop at nothing to "stop the carnage" including taking down a potential threat.
"Sadly I think an attack can happen here, police have thwarted a lot already and people can be radicalised to commit acts of violence," he said.
"Trying to fight against this ideology is like trying to stop the bleeding. How can you change that ideology?"
Speaking on Sunrise this morning, Foreign Minister Julie Bishop said the London attack showed people can use everyday items including motor vehicles and knives to harm innocent people.
"This was part of the ISIS narrative, the Islamic State terrorist organisation was urging its demented followers to pick up a rock or a knife or use a vehicle."
She also said Australia's terror threat remained at probable following the events in London.
Bishop told Sky News all Australians needed to unite in our condemnation of the brutality and savagery of these attacks.
She said since September 2014, when the terror threat was raised to probable, there have been four attacks, more than 60 arrests and 12 thwarted attacks.
"We must work around the clock to keep Australians safe, and that's just not security and law enforcement and intelligence agencies, but the whole community must work together to stamp out radicalisation," she said.
Her comments come a day after Prime Minister Malcolm Turnbull said he had received briefings from the Counter Terrorism Co-ordinator, the Director General of Security and the Deputy AFP Commissioner.
"Australians should be reassured that our agencies are today, as every day, working relentlessly to keep Australians safe," he added.
He also said police will flood public attractions in all the major capital cities.
"We have to be clear-eyed about the risk. It is real. That is why the terror threat level in Australia is set at probable," he said.
Former Prime Minister Tony Abbott said he wants to discuss having the Australian Defence Force as lead agency in complex terror incidents discussed at next COAG.
"A large incident, an incident involving multiple attacks, that's the kind of situation where I believe the military would be the ultimate lead agency," he said today.
Meanwhile, police patrols will be stepped up around popular Melbourne landmarks, including marketplaces, reported the Herald Sun.
Victorian Premier Daniel Andrews said there would be a more visible police presence at places such as Federation Square and Queen Victoria Market in light of the London attacks.
US Defense Secretary James Mattis says America and Australia are united in their resolve to take on terrorists and won't be scared off.
Speaking at the start of the high-level talks with Australia's foreign and defence ministers, Mattis said the US did not take its alliance with Australia for granted and wanted it to strengthen, particularly when it comes to taking on terrorists.
