A man addresses the crowd at a rally protesting the Australian government's treatment of Sri Lankam asylum-seekers in Sydney on July 7, 2014.
The High Court will hear an application on Tuesday to prevent Australia from handing back a boatload of 153 asylum-seekers to Sri Lanka as criticism over the government's hardline policy mounted.
An interim injunction at a late-night sitting Monday temporarily halted the transfer of would-be refugees to Colombo from the boat carrying mostly minority ethnic Tamils reportedly intercepted at sea late last month.
Lawyers argued the transfer was illegal.
It came a day after another vessel was returned to Sri Lanka following a week of secrecy, with local police saying the adults among the group of 41 28 men and four women would be charged with attempting to leave the country illegally.
The crime is punishable by up to two years in jail.
Australian Human Rights Commission chief Gillian Triggs said it was Canberra that had a case to answer, with the screening of asylum-seekers at sea appearing to be inadequate under international law.
The process reportedly involved a four-question interview via video link with the applicants denied the means to challenge it.
"It sounds as though three or four or five questions are being asked by video conference, snap judgements are being made, and they're simply being returned," Triggs told ABC television.
"There is an obligation with international law to have a proper process."
The UN's refugee agency UNHCR said it was "deeply concerned" by the developments, although it did not have enough information about how they were screened to determine whether it was in accordance with international law.
"UNHCR's experience over the years with shipboard processing has generally not been positive," it added in a statement.
Under its policy of not commenting on "operational matters", Canberra has yet to confirm whether the vessel carrying 153 people even exists.
Prime Minister Tony Abbott dogged questions about its fate Tuesday.
"I'm not going to comment on what may or may not be happening on the water, but I do want to assure everyone that what we do on the water is consistent with our legal obligations and consistent with safety at sea," he said.
His comments came as a relative pleaded for news of a three-year-old girl on the missing boat.
"I am desperate to know where my family is. I can't function at all not knowing," the man, reportedly the girl's father, told the Tamil Refugee Council.
"I know all of them would be in very big trouble if sent back to Sri Lanka.
"I want to plead with the Australian minister to stop our pain and let us know what he has done with all the kids and families on the boat."
The US and European Union member states say rights abuses against the ethnic Tamil minority in Sri Lanka have continued even after the civil war ended in 2009.
Australian Greens senator Sarah Hanson-Young said the government seemed to believe it was "above the law".
"The Australian people are becoming sick and tired of the spin, the secrecy, and the danger we're putting these people's lives in," she said.