Prime Minister Malcolm Turnbull must find his compassion and refuse to return asylum seekers to a "devastating" fate at Nauru, regardless of whether a landmark court ruling finds offshore detention is lawful, a prominent child welfare group says.

The High Court is on Wednesday due to rule on whether the federal government has power under the constitution to detain people in Australian-funded detention centres at Nauru and Manus Island.

Should the court find in the government's favour, Immigration Minister Peter Dutton has signalled that about 250 asylum seekers presently in Australia, including 37 babies, would quickly be returned to Nauru. It has raised fresh concerns over the welfare of young children on the remote Pacific island, where asylum seekers and refugees have reported poor health care and living conditions, human rights abuses and regular sexual and physical assaults.

Tensions at Manus Island and Nauru have intensified in the lead-up to the decision, and asylum seekers on Manus have reported a dramatic increase in security despite no protests being planned.

"The atmosphere of the prison these days is very stressful and exciting," detainee Behrouz Boochani told Fairfax Media.

Mr Boochani, who is also a Kurdish-Iranian journalist, said he saw "many people who have not slept during recent days due to the tension and as a result of looking forward to the coming Wednesday".

He said security guards had been seen practicing "how to lie down" and handcuff detainees, adding "they are absolutely ready."

Save the Children's Lee Gordon, who directed the charity's child protection, welfare, education and recreation programs at Nauru for almost two years, said he knew first-hand the "devastating psychological and physical harm that is caused to asylum seekers and refugees living in Nauru".

While there had been some improvement following recent open-centre arrangements, which allow detainees to come and go from the facility, it "can't change the inescapable fact that these vulnerable people are living a life in limbo on a remote island, with little hope for their futures," Mr Gordon said.

"Prime Minister Turnbull must act with compassion, and refuse to return any asylum seekers who have been transferred from Nauru to Australia, regardless of the outcome of the High Court case."

Mr Gordon said the island's residents relied on cargo ships for the delivery of basic essentials such as clean water and nappies. Aside from the "obvious trauma" for parents and children of returning to Nauru, the nutritional needs of babies may not be met and they faced risks such as inadequate housing, safety and hygiene standards, he said.

Mr Dutton and Labor's Immigration spokesman Richard Marles refused to comment on the matter ahead of the court ruling. Mr Dutton's department is yet to respond to claims from asylum seekers about a beefing up of security at Manus and the return of guards to the centre who were involved in the violence of February 2014, when one asylum seeker was killed and scores were injured.

Greens senator Sarah Hanson-Young called on Mr Turnbull to guarantee that no children will be sent back to Nauru.

"The government is threatening these children, saying they can't wait to dump them back on the hellhole of Nauru," she said.

"Most of these children have been going to school here, making friends and trying to build a life in safety. These children are integrating into the community and the government wants to rip them out of it."

Refugee advocates told Fairfax Media that Nauru refugees were hopeful of a court ruling in their favour, and some had tried to sell their possessions in anticipation of being allowed to settle in Australia.

The High Court test case is being run on behalf of a woman from Bangladesh who was brought to Australia from Nauru in August 2014 for medical treatment. Cases for about 200 other people are linked to the court matter.

The case was run by the Human Rights Law Centre. The organisation's director of legal advocacy, Daniel Webb, said the woman, her husband and their one-year-old baby were terrified of being sent back to Nauru.

"This mother just wants what all mothers want – her child to have a good life somewhere safe," he said.

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