Churches across Australia are invoking the historical concept of sanctuary, opening their doors to asylum seekers facing removal back to offshore detention centres.

Key points: 'Sanctuary' concept yet to be tested under Australian law

'Sanctuary' concept yet to be tested under Australian law High Court rejects challenge to the legality of Australia's offshore detention centres

High Court rejects challenge to the legality of Australia's offshore detention centres 270 asylum seekers in fear of being returned to Manus Island or Nauru

The High Court has rejected a challenge to the legality of Australia's offshore detention centres, a ruling that means nearly 270 asylum seekers who came to Australia for medical treatment could be returned to either Nauru or Manus Island.

One of Australia's senior Anglican leaders said places of worship were entitled to offer sanctuary to those seeking refuge from brutal and oppressive forces.

Among the asylum seekers now at risk of being sent back to Nauru or Manus Island are 37 babies.

Advocates for the asylum seekers are running a public campaign to pressure the Government to let them stay in Australia, and now several religious leaders have come forward to offer them sanctuary in their churches and cathedrals.

Anglican Dean of Brisbane the Reverend Dr Peter Catt said he was opening up St John's Cathedral in Brisbane to the asylum seekers.

"Many of us are at the end of our tether as a result of what seems like the Government's intention to send children to Nauru," Dr Catt said.

"So we're reinventing, or rediscovering, or reintroducing, the ancient concept of sanctuary as a last-ditch effort to offer some sense of hope to those who must be feeling incredibly hopeless."

'Sanctuary' yet to be tested under Australian law

Sanctuary is a historical concept and Dr Catt said he would happily risk being prosecuted for offering it.

"Sanctuary was a concept that was certainly alive in the Middle Ages when people could go to a church, and particularly to a cathedral, and claim sanctuary and the church authorities could really grant them safety against the civic authorities," he said.

"It was a way of saying I'm entering into God's territory, away from the civic authorities that are oppressing me, and the oppressors generally accepted that the church could offer sanctuary to people."

Dr Catt said the concept of sanctuary had never been tested under Australian law.

"But my hunch is that if the authorities chose to enter the church and take people away, it would probably be a legal action," he said.

"So this is really a moral stand and it wouldn't be a good look, I don't think, for someone to enter a church and to drag people away."

Immigration Minister Peter Dutton addressed the legality of sanctuary, telling 2GB radio there were a lot of well intentioned people, but even churches were not above the law

"Churches provide lot of assistance to refugees and they feel very strongly about this issue," he said.

"In the end, people have to abide by Australia law, regardless of who they are."

The Australian Churches Refugee Taskforce said the offers of help were coming from across Australia.

Taskforce executive director Misha Coleman said logistically it would be a challenge for the asylum seekers to get to the sanctuaries.

"But if they do, the relevant priest or vicar will manage that situation in a very sort of confidential way," Ms Coleman said.

'Alarming impacts of detention on children'

Among the 250 asylum seekers who now fear being sent back to Nauru or Manus Island are 37 babies. ( Supplied )

Getup human rights director Shen Narayanasamy said there was considerable public support for the asylum seekers.

"We know the Australian public responds with compassion when they can see and hear the fact that human beings are affected by what we're doing," Ms Narayanasamy said.

The Australian Human Rights Commission's new report reveals what Ms Narayanasamy describes as the "alarming impacts of detention on children".

The report is based on interviews and medical testing of children at Wickham Point detention facility, many of whom spent time on Nauru.