Four brothers slipped out of Australia last week to join the fight with Islamic State (Isis) in Syria.

As authorities attempt to contain home-grown terrorists from leaving Australia, the latest recruits from Western Sydney have been labelled "cleanskins" by police agencies because they were completely unknown to intelligence.

The brothers - aged 17, 23, 25 and 28 - told their parents they were going on a holiday to Thailand, according to Sydney's Daily Telegraph.

The siblings are believed to have travelled to Turkey and crossed the Syrian border in the past week.

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Australian Federal Police commissioner Neil Gaughan told a parliamentary committee on Thursday that the four were "not on anyone's radar".

"We got wind of it after the fact, but the fact is there are still people travelling," said Gaughan, the national manager of counter-terrorism for the Australian Federal Police. Authorities were alerted when the family contacted a local community after they failed to hear from their boys.

Immigration Minister Scott Morrison confirmed the family was seeking his help but unfortunately, it was too late.

"Australia has lost yet another four young kids. This is why we are so adamant in trying to prevent people leaving to join these foreign fighters," he said.

"Ginger" jihadist Abdullah Elmir disappeared from his Bankstown home in Sydney's southwest in June and made his way to the Middle East.

The 17-year-old gained notoriety when he declared that Isis would not stop their murderous campaign "until the black flag is flying high in every single land" in a YouTube video.

Gaughan also told the committee he believed that federal police officers needed greater controls urgently, so they were better able to track the movements and interactions of suspects.

Authorities argue they need more tools to monitor suspects and prevent them from being radicalised.

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Though he did not reveal from where or to what destination the group travelled, Gaughan said officers are now virtually powerless to stop them unless they resurface elsewhere.

Gaughan was speaking at an investigation into new legislation that aims to give security agencies power to issue control orders on people who are enabling or supporting terrorism.

He also revealed that authorities were struggling to keep up with the sheer number of people being radicalised. Authorities said the ability to put restrictions on who suspects can communicate with and where they can go would allow them to greatly reduce the chance of an attack on home soil.