Authorities arrested the men -- all Australian citizens of Somali and Lebanese descent -- in police raids across Melbourne, culminating a seven-month counter-terrorism operation, the Australian Broadcasting Corp. reported.
One suspect was charged on a terrorism-related offense and remanded after a court appearance. Three suspects were being questioned and another suspect was in custody on other matters, ABC said.
About 400 law enforcement officers participated in the 19 raids that took place across Melbourne and Victoria, officials said.
Australian Federal Police Acting Chief Commissioner Tony Negus said more arrests were possible.
"There are further inquiries being conducted both in Australia and overseas," Negus said.
Negus said the men were affiliated with al-Shabab, an Islamic militant group that has been battling Somalia's transitional government since 2006 and controls much of southern Somalia, The New York Times reported.
Australia will not raise its terror alert level as a result of Tuesday's raids and arrests, Prime Minister Kevin Rudd said. He said the events served as a reminder of security threats facing the nation.
"The threat of terrorism is alive and well and this requires continued vigilance," Rudd said.