Immigration Minister Scott Morrison says four western Sydney brothers who have joined Islamic State jihadists are now enemies of Australia.
The siblings - aged 17, 23, 25 and 28 - told their family they were heading to Thailand on holiday when they left the country in late October.
Authorities tracked them down in Turkey after being alerted by concerned family members last Saturday when the men failed to contact home.
It is believed they have since crossed the Syrian border to fight with IS militants.
Mr Morrison said they will now have to face the consequences of their actions.
"Once these young men cross over that line they become enemies of Australia and the world," he told reporters in Sydney on Saturday.
The brothers' family is reported to be devastated.
"These four young men have effectively been indoctrinated by ... a death cult," the minister said.
Mr Morrison said authorities had received the tip too late to apprehend them.
The siblings were "clean-skins" and had not been on any watch-lists.
"These were people for whom nothing indicated to date that they might be involved in this type of activity," Mr Morrison said.
He said it was important the community provided information to authorities.
There are now more than 60 Australians known to be fighting in foreign conflicts.