A young girl's involuntary wedding to a man 20 years older than her has been captured with a hidden camera at a Sydney mosque, exposing the disturbing prevalence of child brides in Australia.

In a 7 News exclusive, the footage shows a 14-year-old girl agreeing to wed a man she has only known for a few days as he hands her parents an AU$1480 ($1580) gold necklace in exchange for her hand in marriage.

The celebrant can be heard asking whether both of the parties will live together and live according to Islam - with the young girl reminded she has a duty to obey her husband.

The ceremony took place in a small room in the back of the mosque. Footage of it was played at a Melbourne court last week, with the celebrant now facing charges for his role in the illegal marriage.

The video gives credibility to reports that child brides are becoming more common across Australia - with 7 News reporting they have obtained documents that prove dozens of girls are secretly married to older men.

7 News understands the bride's mother allegedly gave her blessing to her daughter to marry the man.

Sydney police say they are willing to intervene in cases of child marriage, but rely on those within the community to notify them.

Newshub.