A former Don Dale detainee has told the Royal Commission into Youth Detention and Child Protection that a guard at a Darwin adult prison threatened him with rape.

During the fourth day of hearings in Alice Springs, a witness known only as BY, said he was 17 when he was suddenly moved from the Don Dale Youth Detention Centre to the prison.

BY was sent to the adult prison two days after he smashed several windows and camera in the Don Dale facility with a metal pole. He also threw a chair at a senior staff member, which hit him in the shoulder.

He told the commission he stayed in the adult prison for five days, during which time he was kept in a hot isolation cell and only received two showers.

He said he was also threatened by guards.

"At the prison, one of the guards asked me when I would turn 18," he said in a written statement to the commission.

"When I told him, he said, 'Oh, so you'll get transferred back here soon, good, I'll get some of the guys to rape you when you get back'."

Spit hood made detainee 'freak out'

He also spoke of his fear and confusion at being transferred to an adult prison, and said a spit hood was put on him whenever he was transferred within the prison, despite no history of spitting.

The commission heard detainees were locked in wet concrete cells without bedding. ( ABC News: Eleni Roussos )

"Guards came into my cell and they told me to turn around and face the wall. I felt something being put over my head, I couldn't breathe and I didn't know what was going on. I freaked out," BY's statement read.

"I asked what was going on. Someone apologised to me but they didn't take it off.

"I could hardly see out of it and it was really hard to breathe, because it was hot and smelt like plastic."

BY also spoke of an alleged incident in the "back cells" of Don Dale when another inmate began spraying a fire extinguisher around.

He said guards came in, and he was handcuffed and put in a headlock.

"One of them grabbed me behind from around the neck. I said I couldn't breathe, but he didn't stop. I think I passed out," he said.

"We were taken out of our cells and handcuffed to a fence with our hands high above our heads, and we were kept there for more than an hour.

"The fire brigade came and hosed our cells out. The guards took our bedding away. That night we were locked in our cells on the wet concrete with no bedding."

Aboriginal people 'can't get information'

A drawing by detainee BY shows how he was handcuffed to a Don Dale fence. ( Supplied: Royal Commission )

BY said he felt he was being unfairly punished for something somebody else had done.

His grandmother, referred to as CA, also gave evidence to the commission and described it as a "scary moment" when her grandson started getting in trouble with police at the age of 13.

She said he came from a good home, and the family had found it difficult to navigate the justice system.

"As an Aboriginal person, and in this town, you can't get any information at all," she said.

"His mum, being a non-Indigenous, she went in and she got the information. She got answers. The attitude changed when I came to the police station to find out things."

CA, who is a qualified suicide prevention trainer with extensive experience in dealing with children's welfare and mental health, said she would like to see a comprehensive mental health team and Aboriginal support team put together to help young inmates.

The hearing continues.