It might seem small beer, what money can be raised from a suburban backyard in Melbourne.

But consider the price tag for executing the September 11, 2001 attacks in New York and Washington was at most US$500,000 - a paltry amount spent for cost endured.

When small amounts of cash for nefarious purposes can be collected from hundreds, if not thousands, of separate sources across the world, tracking the money and potential dangers becomes exceptionally difficult.

A classified risk assessment produced this month by Australia's money laundering watchdog, AUSTRAC, found charities working overseas were especially vulnerable to exploitation by terrorists' supporters.

Money raised legitimately to help people suffering in conflict zones can be "commingled with funds raised specifically to finance terrorism", a publicly released version of the report warned.

The funds might not be intended for specific plots but instead used for what the report called "the less violent or obvious aspects of a group's operations - daily living expenses, travel, training, propaganda activities, organisational costs, and compensation for wounded fighters or the families and dependants of terrorists who have died".

But the report added important qualifiers: only a handful of charities had been caught up in Australia and the value of funds low.

Authorities try to spot suspicious transfers when dozens of people make separate small deposits in a single account, only for a larger amount from the same account to be sent overseas.

But given the now regular international transactions on eBay or other online shopping sites, the task can be overwhelming.

Deliberate misspelling of customer names when transferring funds to suspect locations is another red flag.

There have been instances where crime - such as credit card fraud by obtaining numbers illicitly from taxi drivers - has also been used to raise funds for terrorists, but AUSTRAC assessed legitimate fundraising activities were more common.

Getting the money into terrorists' hands was complex, often sent via third countries in an attempt to evade detection.

Banks, remittance services and even store credit cards can be exploited.

The report names Lebanon, Turkey, the United Arab Emirates and Jordan as "conduits to route money destined for terrorist groups in Syria".

Couriers are suspected of carrying cash across the border.

Concern has previously been raised over forms of informal banking, sometimes referred to as "hawala", where payments are made across the globe with little documentation.

But the AUSTRAC report said while there was a risk terrorists would abuse the services of dealers who relied on ethnic or cultural links to send funds abroad, "these dealers are also well placed to identify and report suspicious behaviour".