One of Australia's leading doctors has challenged the Prime Minister to prosecute him for speaking out about what he calls "torture-like conditions" in offshore detention centres.

Key points: Paediatrician says long-term detention causes mental health problems, breaches international law, could be considered torture

Paediatrician says long-term detention causes mental health problems, breaches international law, could be considered torture Conditions compared to those in asylums, prisons, concentration camps

Conditions compared to those in asylums, prisons, concentration camps Doctor wrote to politicians calling on them to prosecute him for speaking out under Border Force Act

Professor David Isaacs has been an outspoken critic of the centres, but under the Border Force Act, contractors working in immigration detention — including doctors and aid workers — face two years' imprisonment for revealing details of what happens in detention centres.

The paediatrician visited Nauru in December 2014.

Professor Isaacs said he had now written to Prime Minister Malcolm Turnbull, Immigration Minister Peter Dutton and Labor leader Bill Shorten challenging them to prosecute him under the Act, or to repeal the legislation.

"Long-term immigration detention causes major mental health problems, is illegal in international law and arguably fits the recognised definition of torture," Professor Isaacs said.

"The conditions we witnessed typified those in institutions such as asylums, prisons and concentration camps.

"There was constant bullying and humiliation, and children and adults coming to the medical centre were referred to by their boat numbers."

Professor Isaacs recently published a paper in the Journal of Medical Ethics detailing his experience.

"The average length of detention of the children and families was 14 months and they were still not told when applications for asylum would be processed," he wrote.

"Doctors and nurses have a moral duty both to serve and to speak out."

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According to The Forgotten Children report by the Australian Human Rights Commission and released in 2015, the number of children in detention dropped in the period July 2013 to January 2014 — but the length of detention increased.

Statistics from the Department of Immigration show that the average length of detention for asylum seekers has increased dramatically since September 2013, with the average length more than 410 days.

68 children in offshore detention on Nauru, 79 on mainland

The number of children in offshore detention has dropped from a high of about 1,700 when the Coalition took government.

Latest figures show 68 children are in offshore detention on Nauru, and there are reportedly 79 in detention on the mainland.

Last week, Mr Dutton announced 72 children would be returned to immigration detention on Nauru within weeks.

Claire Hammerton from the organisation ChilOut, which lobbies for children to be taken out of detention, said the trauma for families caught in the system remained intense.

There is a pending High Court judgment that will decide the legality of offshore detention on Nauru.

Australian Human Rights president Gillian Triggs said she was deeply unimpressed at the suggestion of sending scores of children back to Nauru.

"My primary response is one of considerable disappointment," Ms Triggs said.

"The children have been transferred from Nauru to Australia for medical reasons, and they are frankly in despair at the prospect of returning to the conditions and circumstances of their detention in Nauru."

Mr Dutton did not respond to the ABC's inquiries regarding Professor Isaacs' letter.