A surprising number of foreign fighters joining the rebellion in Syria against the regime of President Bashar Assad hail from Down Under.

SALAH AL-ASHKAR / AFP / Getty Images Syrian rebels gather outside a building that they blew up to target a regime sniper taking shelter inside in the Salaheddine district of the northern city of Aleppo on July 10, 2013

Correction appended: July 16, 2013, 5:27 a.m. E.T.

As many as 6,000 foreign fighters from nearly 50 nations have now joined the brutal 2½-year civil war to unseat President Bashar Assad of Syria. The vast majority are veterans from the the Arab Springs of Libya, Tunisia and Egypt. Islamist volunteers from Yemen, Somalia, Iraq, Jordan, Turkey and a few former Soviet republics bolster their ranks.

And then there are the Aussies.

Surprising estimates suggest that Australians now make up the largest contingent from any developed nation in the Syrian rebel forces. There are around 120 French fighters in Syria, about 100 Britons and a handful of Americans — but there are at least 200 Australians, according to a public statement made by David Irvine, director general of the Australian Security Intelligence Organisation (ASIO). The total may appear small, but it is growing rapidly, having doubled since the end of last year — and when looked at as a proportion of the Muslim population of Australia, the figure is startling. The French, British and American rebel fighters are drawn from communities that number 4.7 million, 2.7 million and 2.6 million respectively. The Australian contingent is drawn from a Muslim population of just 500,000, and is causing concern to a government that fears the homecoming of a battle-hardened group of radicalized Islamists when the conflict ends.

In February, Norwegian terrorism expert Thomas Hegghammer released a paper showing that 1 in 9 Westerners who fight in foreign jihadist insurgencies ends up becoming involved in terrorist plots back home. With evidence that more than 100 Australian rebels in Syria are billeted with Jabhat al-Nusra, an al-Qaeda-linked militia, it isn’t surprising that Canberra is becoming alarmed.

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According to Greg Barton, international director of Monash University’s Global Terrorism Research Centre, “It’s very difficult to get hard data or proof on what’s really going on. But the head of ASIO doesn’t come out and make public statements very often, so the fact that he’s talking about this shows how much of a concern this is. ”

Anecdotal evidence suggests that ASIO’s estimate may even be conservative. “The number of Australian fighters in Syria is far higher than a few hundred,” says Jamal Daoud, a Jordanian migrant who stood as a candidate for the New South Wales (NSW) legislature during the last state polls. “I have been talking to community members for years now. So many times people have told me ‘my neighbour is fighting in Syria’ or ‘I am selling our furniture so I can go fight in Syria.’”

In response, Australian counterterrorism operatives have been dispatched to Turkey and Beirut, where they are collecting evidence against a number of Australians suspected of fighting in Syria. Surveillance is also being stepped up back home and a new law was enacted last month making it illegal to be a member of, or recruit for, Jabhat al-Nusra.

Part of the explanation for the Australian presence in Syria, some believe, is sheer opportunism. Barton points out that many of the Australians who have traveled to the war zone come from the northern Lebanese immigrant community — a group that has “experienced disproportionate problems” with organized crime. Nick Kaldas, an Egyptian-born counterterrorism expert who now serves as the NSW deputy police commissioner agrees that there are “people involved in crime who are using the conflict in Syria as an excuse or pretext to carry out more criminal acts.”

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Nonetheless, it is clear that most fighters are motivated by religious extremism — and they and their followers are impatient to start the fight even before they leave Australian shores. The Sunni-Shi‘ite sectarianism of the Syrian conflict is now causing violence in the sprawling suburbs of Sydney. The Shi‘ite owner of a juice bar in the suburb of Auburn says constant harassment and assault by Islamists forced him to close shop. In Bankstown, a chicken shop built by a Shi‘ite city councillor was firebombed two days before it opened. Last year in Lakemba, a 29-year-old man who made pro-Assad comments on Facebook got a knock on the door from a gunman who shot him twice in the legs, while the Sunni strongholds of Greenacre and Punchbowl have allegedly become no-go zones for Shi‘ite.

The violence has spread to other cities. In Melbourne, two prayer rooms belonging to the Alawite sect, a Shi‘ite store and a car yard owned by a Sunni have been firebombed. And in the most brazen attack to date, a group of 40 men stormed the Syrian embassy in Canberra and trashed it as staff huddled in terror.

Moderate Sunnis have also fallen into the firing line. NSW legislature hopeful Daoud claims thugs forced him to abandon his campaign for the seat of Auburn during the 2011 poll because he refused to support the war in Syria. “We could not distribute leaflets or stand in the street without getting attacked,” he says. “I received death threats against my wife and children, calling me a Shia pig even though I’m Sunni.”

It might be Auburn or it might be Aleppo, but it seems that slowly — and unbelievably — more and more Australians are being drawn into the Syrian war.

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An earlier version of this article misspelled the name of the Syrian President. It is Bashar Assad, not Basher al-Assad.

An earlier version of this article misspelled the name of the Norwegian terrorism expert. It is Thomas Hegghammer, not Hegghamer.

An earlier version of this article misstated the name of the research center at Monash University. It is the Global Terrorism Research Centre, not the Global Terror Research Centre.