A Sydney schoolgirl has been arrested by counter-terrorism police for allegedly sending money to a terrorist organisation.

The 16-year-old was arrested yesterday in Guilford in Sydney's southwest with a 20-year-old man she knows.

Police expect to charge the pair with sending funds to an overseas terrorist organisation.

"The 16-year-old girl is not somebody who is well known to us," NSW Police Deputy Commissioner Catherine Burn told reporters in Sydney.


The pair, who were arrested as part of the ongoing Operation Peqin investigation, will face court later.

"It's a very serious charge. It carries a maximum imprisonment of 25 years," Burn told reporters.

The girl's arrest is part of a disturbing trend of young people being "involved in activities that they should really not be involved in at all", Burn said.

The counter-terrorism police had been investigating the man from the start of Operation Appleby.

The pair knew each other but weren't related or in a relationship, Burn said.

She wouldn't confirm if the money the pair had allegedly organised was going to Australians fighting for Isis (Islamic State) overseas.

"It was being sent to a terrorist organisation," she said. "We won't be specifying exactly who or where."

However, Burn did say several people across NSW were involved in raising and sending money overseas to terrorist groups.

AFP deputy commissioner Michael Phelan said the age of some people arrested in counter-terrorism operations was "disturbing".

A lot of information that has led to charges being laid against 14 people under Operation Appleby has come from concerned family and friends.

"We cannot do our job without public support," Phelan said.

He added: "It's not over."