Australian police said on Tuesday they had arrested two people, including a 16-year-old girl, on suspicion of raising funds to support operations of the Islamic State militant group.

The arrests in a Sydney suburb on Monday of the schoolgirl and a man, aged 20, were part of counter-terrorism operations aimed at thwarting attacks by domestic radicals at home and disrupting the flow of funds to foreign fighters overseas.

"We anticipate that both these people will be charged later today and attend court and the charge that we anticipate they will have is one of financing terrorism," New South Wales state police Deputy Commissioner Catherine Burn told reporters.

"We will be alleging that they were involved in obtaining money to send offshore to assist the Islamic State in its activities," she said.

Australia's anti-money laundering agency said in November reports of suspected militant financing had tripled in the past year, with more than A$50mn ($38mn) that could be used to support militants being investigated.

The amounts being sent in this particular case were small and most likely used to help facilitate the travel of foreign fighters into Syria, an Australian Federal Police source told Reuters on condition of anonymity.

"They are doing it usually by credit cards or ATM cards - pretty easy. It's not that much money. Some of it is a very small amount of money ... sometimes less than $1,000," he said.

'Basis for charges'

Authorities believe dozens of Australians have travelled to Syria and Iraq to fight alongside Islamic State militants.

Australia, a staunch US ally, has been on heightened alert for attacks by home-grown radicals since 2014 and authorities say they have thwarted a number of potential attacks, while there have been several "lone wolf" assaults.

Islamic State raises the majority of its funding from oil, kidnapping and other illicit activities in far greater amounts than what is sent by individuals, said Greg Barton, a terrorism expert at Deakin University.

Arrests like those made this week were more valuable in identifying and arresting Australians who may be at risk of further radicalisation than they in putting a dent in militant finances.

"It's more this gives us the basis for figuring out the connection and it also gives us the basis for laying charges," he told Reuters.

Australian Federal Police Deputy Commissioner Michael Phelan said the latest arrests were not connected to any threat of an imminent attack.

In 2014, police shot dead a Melbourne teenager after he stabbed two counter-terrorism officers. Three months later, two hostages were killed when police stormed a Sydney cafe to end a siege by a lone gunman, who was also killed.

A 15-year-old boy fired on an accountant at police headquarters in a Sydney suburb last October and was then killed in a gunfight with police.