The Melbourne man, who the Australian Federal Police alleges was a propagandist for the Islamic extremist group, is accused of sending about 10,000 documents, messages, images, videos and audio files relating to executions and torture.
The 65-year-old retiree allegedly pledged allegiance to ISIS, which stands for the Islamic State of Iraq and Syria, between 2018 and 2020.
The man, a Somali-born Australian citizen, is alleged to have acted on the behalf of the terrorist group without travelling to the territory it controlled during conflict in the Middle East.
His role was significant, police said.
The scale of extremist material the man was accused of spreading put him among the top propagandists on the encrypted messaging platform he used, AFP acting Assistant Commissioner Paula Hudson said.
"The material allegedly shared by this man glorified acts of extreme violence and promoted a terrorist organisation, spreading a hateful ideology that's designed to radicalise others," Ms Hudson said.
The videos, photographs, audio files and documents he allegedly sent contained messages relating to "abhorrent content", including executions, mass murder and torture.
"Terrorist organisations like ISIS rely on online platforms to distribute propaganda and recruit supporters," Ms Hudson said.
"Their content is specifically designed to radicalise individuals, encouraging them to adopt extremist beliefs and commit acts of violence."
The man was arrested at a home in Carlton, in Melbourne's inner north, on Wednesday morning.
Counter-terror officers will also allege that the man shared information about avoiding detection from law enforcement, along with instructional guides on planning and conducting mass-casualty attacks.
He is expected to be charged with being a member of a terrorist organisation, possessing violent extremist material, and using a carriage service to cause offence.
How the man, a Somali-born Australian citizen, came to be radicalised was expected to emerge in court, Ms Hudson added.
The arrest followed an investigation that began with a referral from the Australian Border Force, which intercepted the man and searched his luggage at Melbourne Airport in November 2020 as he left for Egypt.
A search of his mobile phone allegedly turned up suspicious content.
Agents seized the device but allowed the man to continue on to Egypt, where he stayed for a sixth-month period.
Investigators believe the man had compassionate grounds to leave the country, as was required during COVID-19 travel restrictions in place at the time.
There is no evidence the man travelled to any declared no-go zones at the time.
An additional search warrant was executed in 2024.
Police do not allege the man continued to distribute extremist material after his initial interception in 2020.
Authorities believe that more than 200 Australians have joined Islamic State since 2012, some of the more than 50,000 fighters the group was thought to have attracted from overseas.
Having taken control in an area roughly the size of Great Britain across Syria and Iraq at its peak in 2014, IS has since been splintered by foreign adversaries into pockets of Nigeria, Mozambique and Mali.
The man is expected to appear before Melbourne Magistrates Court later on Wednesday.
Police have not ruled out further arrests but said there was no threat to the community.
