An Iranian asylum seeker detained on Nauru has made a report about his alleged mistreatment by Nauruan police to the United Nations human rights agency.

Key points: Asylum seeker arrested, jailed for 30 hours

Asylum seeker arrested, jailed for 30 hours Claims guards urinated into cell

Claims guards urinated into cell Detained after trying see suicidal sister

Detained after trying see suicidal sister Sister attempted suicide twice after alleged rape

The man claims he was detained for more than 30 hours in a bare police cell that was wet with the urine of his guards.

The man has told the ABC and staff from the Office of the High Commissioner for Human Rights that he was arrested and detained without charge after trying to see his sister.

She had just tried for the second time to kill herself after being allegedly raped on the island in May.

The woman has since been transferred to Brisbane in a serious condition while her brother and mother have remained on Nauru.

On the night he was arrested, the 20-year-old man was waiting with his mother for her medication when he heard his sister's scream.

He knows now she had drunk insecticide.

"I just run, because I heard the screaming sound, because I understood that something happened to my sister," he said.

He claims he was stopped by detention centre guards, who grabbed him and forced him to the floor.

The incident was witnessed by a fellow detainee, named Fariborz, who said the guards were very rough.

"Australian officers came and they grabbed his hands. They grabbed him in such a way as if they had arrested a criminal from Daesh group," he said.

"His shoulder blades were being bent, because they had pulled his arms behind him."

Fariborz said Nauruan police arrived soon afterwards.

"His only offence was that he wanted to see his sister for just five seconds. And he kept begging them," he said.

"No matter how much he begged them, they still did not let him see his sister, and they took him to prison."

Guards 'urinated through bars' of cell

The young man claims he was put in a cell without furniture and left there for more than 30 hours while people outside the cell, including guards, urinated through the bars.

"I slept on the urine. Because of some reason that I really don't know why, they urinating into my cell and I used my t-shirt for cleaning and make it dry."

Since then, he has told his story to staff from the Office of the High Commissioner for Human Rights, who were visiting Nauru.

The man asked the agency to investigate and said it agreed to do so.

He said he did not commit a crime and no charges had been laid.

"I just asked to see my sister because she tried to suicide. Because we are family. Because I'm her brother," he said.

"Because when my sister tried to suicide, it's normal that I'm getting upset and I want to see what's happening."

The man said he had been afraid to speak publicly about the matter until now.

"Me and my family, we are completely alone here," he said.

Nauru officials deny allegations

Greens senator Sarah Hanson-Young has also been looking into the family's story.

"If there is any truth to these claims, they are abhorrent and they need and warrant an immediate response from the Immigration Minister," she said.

"This family has been through so much already".

The senator called for an independent Australian investigation into both the man's complaint and his sister's alleged rape.

"I don't believe for a second that it is appropriate for [Nauru security companies] Transfield Services or Wilson Security to be investigating themselves. This is what has happened previously," she said.

"We know the results of these investigations means there's no action. Things often get covered up."

A statement from the Immigration Department said that open and confidential arrangements were in place for detainees to report any concerns they might have.

The Nauruan government denied the man's allegations.