The foiled Melbourne attack has been described as the most concerning in recent years - and it took a complex police investigation to thwart.
As IS continues to terrorise overseas, Melbourne's CBD has also been in its sights.
A propaganda video released last month shows St Paul's Cathedral as a key target.
Now it's been revealed an attack was planned against the Cathedral and other landmarks.
Prime Minister Turnbull says the attack was to take place on or around Christmas Day.
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"It is a time of happiness and joy. These terrorists sought to disrupt it. They have been thwarted, they are in custody. They are no longer a threat to Australians' security."
Australian Federal Police commissioner Andrew Colvin claims the group had moved quickly from an intention to commit the attack, to developing a plan.
"Quite progressed plans we will allege. Now we've heard about Federation Square, we've heard about the Flinders Street train station and St Paul's cathedral. We believe that they were narrowing down exactly what their plan was, but that's all in one part of Melbourne's CBD."
Police and security agencies are being praised for their work in shutting down the attack quickly in a difficult security environment .
Terrorism expert Greg Barton believes averting such an plan would have taken months, if not years, of surveillance.
"The plot was much more concrete, much more real, much more immediate to the point where we can probably assume that if police hadn't been watching these guys, hadn't moved in, we would have seen something happen over the coming days. We may have been just days away from Australia's biggest ever terror attack."
Australian Federal Police commissioner Andrew Colvin says all those behind the plot are in custody.
"Certainly they have associates that we are always interested in, and there's a number of people - and we've said this publicly before - a number of people in this country that we have a remaining security interest in. But in terms of this cell, we believe we've removed the bulk of the cell."
Australia's terror threat level remains at probable.
It was increased in 2014 to indicate a terrorist attack in Australia was likely.
Since then there have been four attacks across Melbourne and Sydney.
The Prime Minister says 12 others have been successfully disrupted - and a total of 57 people have been charged.
"They want to frighten Australians. They want to divide Australians. They want us to turn on each other. We will not let them succeed. Today and overnight our security and police agencies have yet again done their job, their great work."
Opposition Leader Bill Shorten says the few extremists in the community want to disrupt the Australian way of life.
"They win when we give up. We have got great police and security services I'm very privileged in the position I'm in to see some of the reports and the professionalism in which they conduct themselves."
Prime Minister Turnbull says it's a challenge that confronts all Australians, and security agencies alone can't mitigate all of the risk.
"Speak up. Don't ever be afraid of making a call, of letting the police know. Our police and intelligence services keep us safe because people keep them informed."
Australians are once again being urged to go about their lives as they normally would.
Just like Malcolm Turnbull did this morning, when he caught a public ferry to work, much to the surprise of a fellow passenger.
"He said, 'It's amazing that we live in a country where the Prime Minister can get the ferry in to the city, just like everyone else.' I said, 'Well, that's Australia.' And that is Australia. That's what we're standing up for."
