Child abuse detectives said the man was shocked. He believed there was nothing wrong with applying for guardianship, or having an ongoing sexual relationship with the girl because she was his wife.
Police were shocked to learn that this child bride may be only one of at least 250 across the nation, according to a recent report.
Yet NSW Police said last week was the first time they had made arrests in relation to a child bride.
In the wake of the man's arrest, Community Services Minister Pru Goward told 2GB radio that forced marriages of under-age girls may be "quite common".
''I understand there are actually a significant number of unlawful, unregistered marriages to under-aged girls in NSW, particularly in western Sydney, south-west Sydney and the Blue Mountains,'' she said.
But the child abuse squad's Detective Inspector John Betell said it was news to him.
"It is difficult for the child abuse squad to investigate these types of crimes when there is no knowledge of it or reports of it," Betell said.
Last week his detectives charged the man who allegedly married the 12-year-old girl, as well as her father and Imam Muhammad Riaz Tasawar, the Muslim cleric who married the pair in an illegal Islamic ceremony in the Hunter Valley in January.
The 26-year-old man remained in custody charged with 25 counts of having sexual intercourse with a child under the age of 14.
A Newcastle court heard the girl's father allowed his daughter to be married because she was "strong-willed" and "in love". The father also remains in custody.
"The child abuse squad doesn't look at it as an under-age marriage. We look at it that a 12-year-old girl has been sexually abused by a 26-year-old man and that is the investigation we are continuing with,'' Betell said.
Little is known about under-age and forced marriages in NSW, according Matthew Keely, director of the National Children's and Youth Law Centre, and author of the recent report End Child Marriage.
He said much more research was needed to understand the extent of forced marriages in Australia: ''It is an issue which is widely acknowledged as under-reported … the exact figure is unknown,'' Keely said.
Keely's report found child marriage was common in more than half the member countries of the United Nations. Cases of forced child marriage were identified in every state and territory of Australia, across a wide range of religions and cultures.
Despite this finding there are no fact sheets or online resources aimed at educating children about their rights and no guidelines for service providers on the subject.
According to Goward, reliable figures on child marriage were hard to come by because ceremonies involving minors were not legal or formally registered.
"As with the issue of female genital mutilation, which we are toughening up penalties for, under-age marriage is very often kept secret, because the people involved know it is against the law,'' Goward said.
The minister said she wanted to set up a 24-hour national hotline which might make it easier to report and investigate under-age marriages.
Although there was little prosecution of those who arranged under-age marriages on home soil, several cases highlighted the problem of children being sent overseas and forced to marry.
In 2011, a teenager was so desperate to escape an arranged marriage in Lebanon that she took her parents to a Sydney court.
The 16-year-old had pleaded with her mother and father to call the wedding off but they refused.
Just two weeks before she was to marry a boy she had only met once, she applied to put herself on the Airport Watch list.
An order was made preventing her removal from Australia and her parents were forced to surrender her passport. The Australian Federal Police were also ordered to restrain the parents from attempting to make her leave the country.
Federal Magistrate Joseph Harman commended the teenager for bucking her parents' wishes. "This young person is indeed a young woman whose voice can and should be heard," Harman said.
"It is not the right of any parent to cause their child to be married against their will, whether in accordance with Australian law or otherwise."
One year earlier, child protection workers grew concerned when a 14-year-old girl stopped going to school in Victoria. Department of Human Services visited the home of the young girl and learnt she was engaged to be married overseas. She had only seen a photograph of the 17-year-old boy she was to marry.
Her father had stopped her from going to school and she was to be married within two to three weeks. When asked how she felt about the marriage, the teenager said she did not know what to say as she had not met her fiance.
The department made an application to the courts preventing her from being taken out of Australia. A protection worker told the Family Law Court of Australia that it was not in the girl's best interest to travel overseas to marry as she had no understanding of the consequences.
"Furthermore, she would be deprived of a school education, and she may be at risk of sexual exploitation and emotional harm," the court heard.
Justice Nahum Mushin made an order to restrain the child's parents from removing her from Australia and placed her on the Airport Watch List. The parents were also restrained from applying for a passport for her until she turned 18.
"In my view, a 14-year-old child would not have the understanding of the significance of marriage which would be attributable to an adult,'' Mushin said.
"To be taken overseas for the purpose of marriage in any circumstance is contrary to her welfare."
The 26-year-old man charged with marrying the 12-year-old bride is expected to apply for his release from custody within two weeks.
A Lebanese man was "determined" to marry a 12-year-old girl he saw in a mosque, despite being told it was illegal.
The 27-year-old, who cannot be named for legal reasons, was sentenced to a maximum 10 years' jail in the Downing Centre District Court on Friday, after pleading guilty to the persistent sexual abuse of a child.
Judge Deborah Sweeney said the man, who arrived in Australia on a student visa in June 2013, asked two religious leaders at a Hunter Valley mosque to arrange the marriage. They declined, clearly telling him it was illegal.
Undeterred, the man embarked on a "determined and deliberate pursuit of the girl" and with the help of one of her relatives, organised for a ceremony to take place at a private home on January 12, 2014.
In setting a non-parole period of seven-and-a-half years, Judge Sweeney said "neither his cultural or religious beliefs excuse his offence".
The court heard he believed a Muslim girl could marry once she reached puberty.