A Sydney teenager had been brainwashed by a man she intended to marry when she helped transfer almost $20,000 to Islamic State fighters, a court has heard.

The 17-year-old, who can't be named for legal reasons, wanted to impress the older man when she helped arrange several Western Union cash transfers in 2015 and 2016, her lawyer Zemarai Khatiz told the Parramatta Children's Court on Friday.

"She had romantic feelings toward him, he had told her that he wanted to marry her," Mr Khatiz said.

The schoolgirl pleaded guilty in November to knowingly collecting funds for a terrorist organisation after she helped transfer $17,850.60 to the man in Syria when she was 16 years old.

He told her the money would help the "mujahideen" or jihad fight the kafir, the Arabic term for unbeliever, the court heard on Friday,

Mr Khatiz said the girl had been brainwashed and manipulated by the man, who met and proposed marriage to her when she was 14. He was nine years older.

"Her life had hit rock bottom, she didn't have friends, she didn't have a father, problems at home, and the influencing occurred in that context," he said.

But prosecutor Troy Anderson said the girl knew what she was doing and where the money was going, and he is pushing for her sentence to be decided in the NSW District Court where she will face a tougher penalty.

The girl has so far had her matter heard in a children's court where the maximum sentence is two years imprisonment - while the maximum sentence in the district court is 15 years.

Mr Anderson argued a heftier sentence was warranted given the severity of the girl's crime and her level of involvement over many months.

"This is a very serious offence, and this is a serious example of it," Mr Anderson said.

Magistrate Estelle Hawdon is expected to hand down her judgement next Friday.

AAP

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