Australia has concerns that its citizens are fighting alongside Sunni militants in Iraq and Syria, including with the radical Islamic State of Iraq and the Levant (ISIL) group.
"The important thing is to ensure that as far as is humanly possible they don't come back into our country," Abbott told Sydney radio station 2GB.
"And if they do come back into our country they are taken into detention because what we can't have is trained killers -- who hate our way of life, who hate us -- making mischief with the potential to cause mayhem in our country."
Abbott said more than 100 Australians had travelled to Syria and Iraq, and some had been involved with "this murderous, murderous Al-Qaeda splinter group," a reference to ISIL.
"Be in no doubt that some individuals from this country are now participating in acts of barbarity in Iraq," Abbott added to journalists.
"These people should have no place in our country and we will do our best to keep them out."
He said earlier that most were dual nationality citizens.
The European Union warned earlier this year that the number of young European Muslims going to fight alongside extremist groups in Syria and elsewhere is fast growing, and governments fear they will pose a security risk if they return home.
Abbott said authorities were keeping a careful watch on as many of the Australians who had travelled to conflict zones as possible.
Australian Foreign Minister Julie Bishop said Thursday she was "deeply concerned" about the threat Australian fighters could pose when they returned home, adding she had cancelled a number of passports on the advice of intelligence agencies.