The academics wrote to the government before Christmas, and again on Monday night, urging the government to free children detained in Australia and on Nauru as a matter of urgency.

Signatories included experts on child psychology, human rights, public policy and the law, from universities across the country.

"We are concerned about the serious deleterious effect this [detention] has on the physical and psychological welfare of children, who in effect are being punished in the absence of guilt on their part of any kind and outside the normal legal, child protection and welfare frameworks within which their situation should more properly and appropriately be addressed," the academics wrote.

Addressing their letter to all federal politicians, the group said the detention of children was in breach of Australia's international law obligations, and urged the government to release children into the community while their asylum claims were being assessed.

Only one government MP responded. In a curt response sent on Christmas Eve, Liberal Senator Eric Abetz thanked the letter's author, the University of Melbourne's Professor Philomena Murray.

He went on: "To make such a request so close to Christmas shows a sad disassociation from practical considerations. Further, I note that when the Coalition came to Government, there were about 2000 children in detention and that has now been reduced to well under 100. Further, I note that when Labor came into Government, there were no children in detention. This is indicative of the success of our border protection policy."

Immigration Minister Peter Dutton told Sky News on Monday there were 78 children in detention in Australia, and 72 of them would be sent soon to Nauru.

But asked for the number of children detained in Australian mainland facilities and Nauru, the Department of Immigration and Border Protection pointed to December 30 figures showing 91 children were detained on the mainland – 59 in so-called "alternative places of detention", which can include correctional facilities, hotels and foster care; and 32 in residential or transit housing. A further 68 children were being detained on Nauru.

Professor Murray said no Labor MPs responded to the letter.

On Monday night, Professor Murray wrote a second letter to Mr Turnbull, reiterating the group's concerns.

"We appreciate that the numbers of children in detention have been reduced," she wrote. "While numbers are important, one child in detention is one too many."

Mr Turnbull's office was contacted for comment.