A leading Australian doctor says he will risk his license to keep speaking out about “torture conditions” in offshore detention.

Professor David Isaacs is one of many contractors working in immigration detention that face two years' imprisonment if they reveal details of what happens in the centres.

But since his visit to Nauru in December 2014, Professor Isaacs has become a vocal opponent of the secrecy imposed under the Border Force Act.

He has written to Prime Minister Malcolm Turnbull, Immigration Minister Peter Dutton and Labor leader Bill Shorten calling for them to prosecute him under the Act or to repeal it.

“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.

“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.”

Stats show the number of children in detention has dropped, but the length of their detention has increased.

There are currently 68 children in offshore detention on Nauru, and 79 in detention on the mainland.

But Mr Dutton has recently announced that 72 children will be sent back to immigration detention on Nauru within weeks.

Australian Human Rights president Gillian Triggs says she is deeply unimpressed.

“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.”