The scandal has focused new attention on the management of Cambodia’s orphanages and children’s charities and the deception used by some to raise money.

Long Ven, the 33-year-old director of the Underprivileged Children School in Siem Reap, has been charged with procuring children for the purpose of prostitution and faces up to 10 years' jail if convicted.

Police have not made public the nationality of the volunteers alleged to have abused students.

“We will find the foreigners but I cannot say more because this is a police secret,” Duong Thavary, a provincial anti-human-trafficking police chief told The Cambodia Daily.

Former Victorian policeman James McCabe, who helped investigate the case, told The Phnom Penh Post that more charges against Mr Long Ven, are expected.

“This gives us time to go forensically through the roughly 6500 messages on Facebook and the laptop computers and cameras,” said Mr McCabe, who works for the Cambodian Children’s Fund.

Police said alleged victims include two 17-year-old boys who were among nine students found during a raid on Mr Long Ven’s house.

The school has closed and no one there replied to a request for comment.

Its website appeals directly to foreign visitors for funds to “make sure that most underprivileged Cambodia children are not denied an opportunity acquiring knowledge”.

Mr Long Ven, who is known as “Waha”, posted on the school’s website that his “bragging rights” included to “teach morality”.

The school includes boarding facilities where foreign volunteers come for weeks at a time, many of them well-meaning donors or gap-year students, including some from Australia, which has the greatest involvement in Cambodia’s orphanages of any nation through Australians running them directly, volunteering or donating.

While many of Cambodia’s 600 orphanages are well run, there is growing criticism in Cambodia and other developing countries about so-called “orphan tourism” and “volunteer tourism” where thinly disguised businesses exploit both tourists and volunteers.

Sometimes children are kept to look poor and go out to tourist areas to tout for support, welfare groups say.

Fairfax Media reported last year that about 72 per cent of the 10,000 children living in Cambodia’s orphanages have parents, and studies show that in all but exceptional cases they would be better off living in their own communities.

Foreign donors are keeping thousands of children who have parents in orphanages and institutions for years, social workers say.

Friends International, a leading child protection group, warns that volunteer tourists do more harm than good.

“Although they mean well, they’re fundamentally supporting flawed institutions,” said James Sutherland, spokesman for Friends International in Phnom Penh.

Groups such as Friends International have called for compulsory background checks on tourists visiting Cambodian orphanages and schools after several other cases of child abuse.

Cambodia’s aid worker community has been shaken by the resignation from her charity of Cambodian anti-sex-trafficking celebrity Somaly Mam over revelations she fabricated a child-sex slave victim’s story as part of a deception aimed at helping to raise millions of dollars.

The Somaly Mam Foundation is linked to the Sydney-based Project Futures, which campaigns to end human trafficking.

Project Futures’ chief executive Stephanie Lorenzo says her organisation believes it can continue to support Somaly Mam’s project in Cambodia despite her fall from grace “but only if we are comfortable that the money will continue to go towards helping victims and survivors of sex trafficking”.