Nauruan police are investigating the abuse of an Iranian asylum seeker as new details emerge of her violent sexual assault last week.

The graphic details come as allegations of sexual abuse continue to swirl around the Australian detention centre on the Pacific island and as the Department of Immigration prepares to come under the scrutiny of the Royal Comission on child sex abuse.  

The 23-year-old asylum seeker has told Fairfax Media that she was leaving the detention centre on a day release last Saturday when she claims that someone jumped out from behind her, put his hand on her mouth and forcefully pulled down her top.

She said she was then forced to give violent oral sex for over an hour as her assailant bit her breasts and shoulders.

After her attacker fled, the naked, distressed and disorientated woman managed to find a local police unit who put her in the back of their truck.

"They didn't take me back to the police station, they decided to take me with them to watch the fireworks for Nauru's constitution day," she said in a statement. "It was not until hours later that the police asked for an interpreter".

She claims that she was asked to repeat the story of her assault three times without receiving medical attention.

Last week Nauruan police said that there was no suggestion that anyone had assaulted her.

In a statement they said the woman was found walking in a nearby street naked after failing to return to the the detention centre on Saturday night.

"There is no injury to her or any sign of other physical force or trauma," the Nauru Police Force said.

In an about face last week, the police force have since launched an investigation and promised to "find the person who did this".

The claims come in the same week that the former chief justice of Nauru, Geoffrey Eames said he had no confidence in the ability of the Nauruan police force to investigate allegations of sexual abuse.

"In Nauru there are very grave doubts as to whether the rule of law is a principle which holds much weight at all within the government," he told the ABC. "The Australian government cannot wash its hands of this and say it is a matter for the local police".

An ongoing senate inquiry into Nauru which began last week has also heard shocking allegations of rape and the sexual assault of children. The evidence comes on the back of the Moss Review which reported allegations of guards trading marijuana for sexual favours and inappropriately entering shower facilities.

The committee chair, Labor's Alex Gallacher, said he found Transfields ignorance of logistics and conditions on Nauru "extraordinary" after its executives repeatedly took questions on notice during the inquiry.

The head of the Immigration Department Mike Pezzullo has described several situation reports as "concerning".

He said the cases were being investigated either by the Nauruan police or Australian authorities.

Last week Fairfax Media revealed that the Royal Commission into child sex abuse had also begun investigating the immigration department, but ruled out Nauru and Manus because they fall outside of its jurisdictional powers.

On Monday, the Nauruan government also came under fire from around the world for its decision to block Facebook. The United Nations special rapporteur on freedom of opinion and expression, David Kaye, called on the government of Nauru to lift the ban on the social media site, which has been outlawed since April to "protect the national culture".

"These restrictions are designed to prevent asylum seekers and refugees in the country from sharing information on their situation," he said.

The senate committee on Nauru is due to report its findings by June 15.