Isis jihadi, 18, from Melbourne

"Too many Australians, it seems, are being brainwashed online by this death cult," says Australian Prime Minister Tony Abbott.

Australian Prime Minister Tony Abbott says his Government is doing all it can to stop Australians travelling overseas to fight with extremists after revelations a Melbourne boy had been recruited by Islamic State.

An Isis fighter declared a "white jihadi" by the British press in December has been identified as an 18-year-old Melbourne man called "Jake" .

He was a good student at Craigieburn Secondary College then dropped out last year after converting to Islam.

The revelations come just days after two teenage brothers were stopped at Sydney airport by customs officials who suspected they intended to join the conflict in the Middle East.

The youths, aged 16 and 17, were detained on Friday after they aroused the suspicions of two customs and border protection officers, Immigration Minister Peter Dutton said.

The teenagers were referred to the customs counter-terrorism unit, which determined they were travelling without the knowledge of their parents. The brothers were allowed to leave the airport with their parents and were issued with court attendance notices.

"Too many Australians, it seems, are being brainwashed online by this death cult," Abbott said. "It's important we do everything we can to crack down on this."

- AAP