Sydney maths tutor accused of child sex

A Sydney maths tutor accused of molesting his students was a man with two faces, prosecutors say.

The first was a trusted member of the Vietnamese community.

But the second allegedly used that reputation to get inside parents' homes where he would abuse their children, a court has heard.

Quy Huy Hoang, 67, has pleaded not guilty to 12 counts of abuse including sexual intercourse with a child and indecent assault.

The crimes were allegedly committed against his pupils, aged under eight, in three family homes between 2007 and 2014.

The Greystanes man, also known as Peter, allegedly molested a girl between the ages of three and five by digitally penetrating her, kissing her on the mouth and making her "pinky promise" not to tell her mother.

In his opening address of the trial at Downing Centre District Court, crown prosecutor Paul Leask said Hoang used his good reputation to deceive parents.

"He was living a double life," Mr Leask said.

"He was two men: one seen, one trusted, the other ... with an entirely different agenda."

Mr Leask said the teacher brought lubricant to his lessons, for "no other purpose but sexual intercourse".

It is alleged Hoang transferred KY Jelly into a small Blistex lip balm container and carried it to class in his briefcase, which will be tendered as evidence.

Defence lawyer Todd Pickering urged the jury to have an open mind and said Hoang, who remains on bail, denies any sexual contact with the children.

He stressed the reliability of the young witnesses' accounts would be a fundamental issue in the case.

The trial continues.