Australian church officials offering sanctuary to asylum seekers have no legal basis for their actions and could face criminal sanctions.

Ten Anglican churches and cathedrals have invoked the ancient Christian tradition to offer protection to the 267 people - including 37 babies - facing imminent transfer to Nauru after the High Court on Wednesday upheld the legality of the government's offshore processing regime.

The movement is being led by the Anglican Dean of Brisbane, Dr Peter Catt, who has declared his St John's Anglican Cathedral a place of sanctuary.

Legal experts said the image of fugitives seeking sanctuary in churches and abbeys has been popularised in literature and film but it is "more myth than legal reality" in modern times.

But University of Sydney professor Mary Crock, an expert on refugee law, said that, at a political and symbolic level, it was a "really powerful gesture" and she hoped the churches would follow through with their offer.

"The law can't help any more. We're down to getting out on the streets and using Twitter and modern media. We have to stand up," Professor Crock said.

"This gesture really highlights the gulf that has opened between the law in Australia and morality. We're just a country that is a human rights wasteland. Sadly, it is pointless to go to the High Court."

The institution of sanctuary was "swept away" in England in 1624 when a statute was passed banning the practice, English legal historian Sir John Baker has written.

But the practice was recognised even before the English common law. Professor Constant Mews, the director of the Centre for Studies in Religion and Theology at Monash University, said it pre-dated Christianity.

"Even the pagan temples of Greece and Rome had the sense of the sacred; that this was a space that was under the protection of the gods," Professor Mews said.

"In the Middle Ages, sanctuary developed as a way particularly of those seeking refuge from unjust imprisonment."

Professor Mews said the practice was "abused, in the eyes of the authorities".

"By the late Middle Ages, criminals and debtors were seeking sanctuary just as a way of escaping from the law.

"That led to the English government in the 16th and early 17th century clamping down on that abuse of the sanctuary, but there was a still a recognition that churches did constitute a sacred space. It was a place where armed guards [and] security forces could not go in with impunity."

In modern times, the concept has been invoked by churches and asylum seekers even though it does not have a legal basis.

Canada's National Post reported in 2012 that the practice was still alive, including in the case of two Nigerian students who sought sanctuary in a church in Regina, the capital of Saskatchewan.

University of NSW law professor George Williams said: "Many people seem to think they must have some legal basis for [sanctuary] ... because it's popularised in movies and all sorts of things.

"But it was abolished long before the British even arrived in Australia in 1788. It exists now more in the realm of myth than reality in law."

Australian churches were "really asserting a moral claim" and potentially calling the government's bluff following the High Court decision that found offshore detention was lawful.

But he said the government would be likely to take action against church officials because if it did not "it would be setting a very significant precedent for the churches being able to operate outside of the law".

with Tim Young, Marissa Calligeros and Adam Gartrell