In reacting to the events in London, Prime Minister Malcolm Turnbull set out to assure.
"The terror threat level is set at probable. But we will never ever let the terrorists win. We will defeat and destroy them on the battlefield. We will defeat and defy them at home."
At the Sydney Opera House concrete barriers are being installed as a precaution and in central Melbourne the Australian flag was flown at half-mast.
The federal opposition leader Bill Shorten says there's strong bipartisan support to counter the terror threat.
"Despite the political domestic debates in Australia, all of us in Australian politics are united in our hatred and opposition of terrorism and the terrorists should know whenever they are, that they will never divide this country."
It has been revealed that more than 500 Iraqi and Syrian refugees in the past year have been refused entry to Australia because their names were on an international security watch list.
Thirty of those were also on ASIO's national security terrorism watch list.
The Immigration Minister Peter Dutton says it was appropriate for the government to take its time to screen the refugees.
"The Labor Party had advocated that we should bring people in very quickly and if we had have done that we wouldn't have detected these people and I think there would have been significant consequences in our own country."
ASIO's Director General Duncan Lewis says the potential return of so-called foreign fighters with the anticipated collapse of the self-proclaimed Islamic State in Syria and Iraq is providing its own set of challenges for authorities.
"We can anticipate the effective and appropriate management of returning foreign fighters to be a polarising and a dividing issue in Australia. We have plans in place, they are currently being executed."
The London attack is being used by some to reiterate their positions on immigration.
One Nation leader Pauline Hanson wants a ban on Muslims migrating to Australia.
"I have my own hashtag and you won't need to be praying for this place or this place. It's pray for a Muslim ban. That is how you solve the problem."
And Queensland MP Bob Katter has repeated his calls for visa bans from some African and Middle East countries.
