At an undisclosed location, in a windowless room, Australian cyber spies snooped on Islamic State, plotting an online attack that would ultimately cripple the terror group's propaganda unit.

The 2016 operation was integral in trying to stop the spread of Islamic State's indoctrination but has remained classified until now.

The ABC has revealed Australia's participation in an international effort that's been described as the largest cyber offensive in US military history.

A so-called scorched earth attack devasted Islamic State's ability to recruit fighters.

Here's how they did it.

Why hack Islamic State?

As Australia and its allies fought Islamic State from the ground and in the air in the Middle East, it became clear that the terror group's online skills were helping to build its caliphate.

At its peak, Islamic State had at least 100 people working for its propaganda unit. They included IT specialists, graphic designers and editors.

The team produced glossy digital magazines in several languages that outlined ways to launch an attack and published high quality videos to recruit members from across the world with great success.

Sitting at computers, Islamic State fighters proved just as important as those carrying guns and making explosives.

"A lot of the individuals who worked on the virtual caliphate had the same salaries and titles as military commanders on the ground," said Lydia Khalil, a research fellow at the Lowy Institute.

How was Australia involved in hacking Islamic State?

The Australian Signals Directorate (ASD) is one of the nation's most secretive organisations and is tasked with intercepting foreign communication and cyber warfare.

The United States drafted a team of 20 from within ASD to join a top-secret mission, called Operation Glowing Symphony, aimed at hacking into Islamic State's propaganda computer network.

The Australian team included hackers, or "offensive cyber operators" as they're officially called, counter terrorism experts, linguists, intelligence experts and technical specialists.

The team spent four months preparing for a cyber-attack on Islamic State, during which they spied on the terror group's propaganda unit members.

"You need to know how they will respond when we conduct operations against them, to inform your tactics of how you will go back against them pending your responses," said Sarah, an ASD hacker who led the Australian operation, but whose identity cannot be revealed for security reasons.

Sarah worked alongside a team of counter terrorism experts, cultural experts and behavioural analysts to hack into Islamic State. ( ABC News: Emma Machan )

What was involved in the hack?

Australia and its allies found 10 entry points to hack into Islamic State's online network.

The ASD won't reveal how hackers broke those doors down and gained access into the propaganda unit's systems.

But the techniques can include working out a username and password, by sending phishing emails, or by exploiting security flaws from people not updating their computer systems.

Those months of snooping were finally put to the test when US agents issued a code word, an undisclosed a football team name, to launch a global cyber offensive.

Working in a windowless room in Canberra, ASD's cyber fighters tapped away at their keyboards, targeting a list of targets pinned to the wall.

For 12 hours they accessed accounts, locked Islamic State members out, stole the contents and deleted backups of the files.

The cyber-attacks continued for a week, crippling the propaganda unit's systems.

What did they take from Islamic State?

The exact nature of what ASD secured in the attack remains highly classified.

But the ABC has revealed the Australian team scooped up terabytes of data from the Islamic State network.

That included everything from photos and videos to documents.

The hack would have left Islamic State fighters bewildered when they tried to log into their computers.

"[It was] like scorched earth," ASD's Ben Staughton told the ABC in an exclusive interview.

"You no longer had access to email, to your virtual private networks, to your media production capabilities."

Operation Glowing Symphony hacked into Islamic State's online system, undermining the group's ability to produce propaganda and spread information. ( ABC News: Emma Machan )

Does that mean Islamic State is no longer a threat?

The terror group was locked out of producing the material it had used to recruit fighters around the world.

It left Islamic State unable to produce high quality material and spread its propaganda in a timely manner.

But it didn't stop Islamic State from trying to rebuild its efforts.

"You did see a difference, but you didn't see a complete eradication of Islamic State's online presence either," Ms Khalil said.

She said Islamic State had proven its ability to recover, albeit from a weaker position, each time it was attacked.

"There would be a takedown, Islamic State would recover slightly, but each time they were starting lower and lower on the ladder."

The ABC is aware that further cyber-attacks against Islamic State have been launched since Operation Glowing Symphony, but the ASD has refused to comment.