Greens senator Sarah Hanson-Young has accused Immigration Minister Peter Dutton of mentally torturing a child by keeping her in detention on Nauru.

The five-year-old girl is currently being held along with her parents in Darwin because of her father's poor health, but the family has been told they will be returned to Nauru once doctors have seen the man.

The girl has been diagnosed with post-traumatic stress disorder (PTSD) because of her experiences in the detention centre, and her father said her mental health was deteriorating.

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A report from Save the Children said the child was displaying sexualised behaviour on Nauru.

"Yes, she was under [the] impression that we would be also taken to the Nauru because she could hear them, she could hear the voices from outside and was begging me to stop them from taking us back to Nauru," her father said.

"I knew that if they would have returned me, returned us back to Nauru, my child's misbehaviour would be even worse."

Senator Hanson-Young has written to Mr Dutton asking that he heed the advice of experts and move the child out of detention.

"The minister is torturing this little child and I don't use those words lightly," she said.

"We know that the detention has caused these issues for her. Her mental health has deteriorated."

A drawing by the girl depicted a person with their lips sewn together, lawyer John Lawrence said. ( Supplied )

The move out of Darwin and back to Nauru will happen despite mental health experts recommending the child and the family stay together and be housed in the community rather than detention.

Psychiatrist Peter Young worked until last year for International Health and Mental Services, a company contracted by the Government to provide mental health assessments to people in detention.

He said the situation was a familiar one to health professionals working in detention.

"Unfortunately ... the department will not take medical advice about what should be done in the best interests of managing people's health conditions," he said.

In a statement, Mr Dutton said Senator Hanson-Young's statement was "repugnant".

"That's a repugnant statement, even beneath Sarah Hanson-Young, particularly given that 1,200 people died at sea while the Greens were in government with Labor," he said.

Meanwhile, lawyer John Lawrence is planning action in the Federal Court to prevent the family being returned to Nauru.

"Our Government's plan for that young girl [is for her] to be returned to where she has come from, having already spent over one year in a jail facility which would be unsafe for an adult male Australian criminal," he said.

"That is why we are injuncting (sic) the Department of Immigration, to prevent this happening, and we are also seeking a remedy that she be released, with her parents, and put into the custody — via a community visa — of uncles who live in Sydney."