Facebook has been criticised for allowing a 16-year-old child bride in South Sudan to be auctioned off to the highest bidder on its platform.

Key points: The 16-year-old girl became the winning bidder's ninth wife

The 16-year-old girl became the winning bidder's ninth wife Critics likened the Facebook auction to "latter-day slave markets"

Critics likened the Facebook auction to "latter-day slave markets" Child marriage is illegal in South Sudan but remains widespread

Her father received 500 cows, two luxury cars, two bikes, a boat, some mobile phones and $13,800 in exchange for his daughter.

It is the largest dowry ever paid in the civil war-torn country, according to the Government.

Five men bid on the girl, including high-ranking South Sudanese government officials, according to children's rights organisation Plan International.

The post advertising the girl was first shared on October 25, but it took Facebook until November 9 to remove the post — six days after she was married off to the winning bidder.

She became the man's ninth wife.

George Otim, country director of Plan International South Sudan, likened the auction to slave markets and slammed Facebook for allowing it to take place on the company's platform.

"This barbaric use of technology is reminiscent of latter-day slave markets," he said.

"That a girl could be sold for marriage on the world's biggest social networking site in this day and age is beyond belief."

Mr Otim called on South Sudan's Government to investigate and suspend officials who took part in the bidding, while also encouraging girls who were forced into marriage to report their case to the police.

Facebook defends its safety measures

In a statement to the ABC, a Facebook spokesperson said the post violated its policies and was quickly removed once the company learned of it.

"We're always improving the methods we use to identify content that breaks our policies, including doubling our safety and security team to more than 30,000 and investing in technology," the statement read.

"Any form of human trafficking — whether posts, pages, ads or groups — is not allowed on Facebook."

While the company repeatedly said it works around the clock to remove prohibited content, it has not directly answered why it took just over two weeks to remove the original post.

Child marriage rife in South Sudan

According to UNICEF, more than 50 per cent of South Sudanese girls are wed before their 18th birthday, despite that being the legal age for marriage.

While South Sudan's Government condemns the practice of child marriage, it says it cannot regulate communities' cultural norms, especially in remote areas.

"You can't call it bidding as if it was an auction. It's not bidding. If you see it with European eyes you'll call it an auction," Government spokesman Ateny Wek Ateny said.

"You have to see it with an African eye, as it's a tradition that goes back thousands of years. There's no word for it in English."

High levels of poverty, conflict, instability, low levels of literacy, and gender inequality in education have fuelled child marriage in South Sudan for years, according to campaigners.

Many communities see child marriage as a way to protect girls from pre-marital sex and unwanted pregnancies, or to exchange them for dowry of resources such as cattle.

Women's rights organisation Equality Now welcomed Facebook's removal of the post, but said the auction could create a dangerous precedent whereby other families may try to use social media to seek larger dowries for their daughters.

"Facebook has a responsibility on securing and protecting the rights of women and girls," said Judy Gitau, a lawyer at Equality Now.

"They need to put sufficient resources into monitoring what is on their platform."

ABC/wires