A cross-section of Australia's most prominent Christian leaders is imploring the nation's politicians to "do the right thing" and release asylum seeker children from detention before Christmas.

After lobbying MPs from all parties last week, they have published an open letter calling for children and their families to be given "the gift of freedom" before the Parliament adjourns on Thursday for the year.

"In the next week, you have the opportunity to decide whether to release children and their families from immigration detention," they write. "This is a critical moment in the history of this nation."

It is not clear whether the Turnbull government will bring on legislation amended by non-government senators last week for debate in the House of Representatives in Parliament's last week of the year.

The plight of children in detention was raised this week by Gillian Triggs, president of the Human Rights Commission, who told a Melbourne audience Australia continued to hold in closed detention about 2044 people, including 113 children. There are 934 males on Manus Island and 631 refugees on Nauru, including 92 children.

"Most have been held for well over a year in conditions that have been criticised by UN Rapporteurs as breaching the Torture Convention and constituting 'calibrated cruelty'," Professor Triggs said.

On Nauru on Friday, a sole demonstrator stood on a crane with a hand-painted placard saying, "World: kids in Nauru need help".

Aside from providing for the release of all children in mainland detention centres, the legislation introduces mandatory reporting of abuse. It also reverses legislation making it a criminal offence for public servants and contractors, including doctors, to disclose knowledge acquired in detention centres.

Sister Anne Lane, spokesperson for the leadership group, told Fairfax Media there was "a wonderful sense across all parties that it's time to do this".

But she conceded that it could be difficult for those backing change to prevail in their party rooms. Several of the religious leaders met this week with Greens senator Sarah Hanson-Young, Liberal Craig Laundy, independent Cathy McGowan and Labor MPs Melissa Parke and Anna Burke.

Their letter is published in a half-page advertisement in Saturday's Sydney Morning Herald. Among the signatories are Archbishop of Melbourne, Denis Hart; Primate of the Anglican Church of Australia, Archbishop Philip Freier; President of the Uniting Church in Australia, Stuart McMillan; congregational leader of the Sisters of Charity, Sister Clare Nolan; and Auxiliary Bishop of Sydney, Bishop Terence Brady.

"This is the best opportunity the government has had to get children out of detention," Senator Hanson-Young said. "I hope the Turnbull government doesn't stick to the playbook of cruelty and injustice towards innocent children that epitomised Tony Abbott's leadership."

Sister Lane said the issue had been discussed at a cross-faith meeting of around 40 Christian, Muslim and Jewish leaders in October. "I think there is a sense among all of us that there has been a shift in the community and we need to make doubly sure our politicians are aware of it," she said.

"For some of the children, this will be their third Christmas in detention and we can see no reason why they can't be released before spending another one in detention. There's certainly a willingness around our communities to assist them in many, many ways."

Sister Lane said the religious leaders were similarly concerned about the plight of men on Manus Island, but believed the immediate focus should be children and their families.

"Perhaps when people see that they are no danger to society, we have a chance to further the conversation."

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