By Jane Wardell
SYDNEY, Aug 21 (Reuters) - Australia's highest court set a date on Thursday for a full hearing into how a group of Sri Lankan asylum seekers were detained at sea, a case that will test the government's authority to pursue its secretive immigration policies.
Lawyers for the group of Tamil asylum seekers said it was likely the United Nations would seek to join the case, a highly unusual step they said showed the level of international concern over Australia's "Operation Sovereign Borders".
Justice Kenneth Hayne, who has said the case appeared to be unique in the world and raised serious questions about how far Australian power extends, set an expedited two-day hearing before a full bench of the High Court for Oct. 14-15.
The boat carrying the 157 Tamil asylum seekers was intercepted in late June and held by Australian authorities at sea for weeks. The group's lawyers argue that the their detention and the government's plan to send them to Sri Lanka or back to India, from where they had left, were illegal.
The fate of the group has highlighted Australia's immigration policy, in which boats carrying would-be asylum seekers are intercepted at sea and turned back. The policy has been condemned by the United Nations and human rights groups.
Lawyers representing the asylum seekers indicated that the UN High Commissioner for Refugees (UNHCR), which in July expressed "profound concern" over the case, and the Australian Human Rights Commission (HRC) would seek to join the case. The U.N. would not be a direct party to the case but could offer legal opinion, testimony and text evidence.
"What Australia does on the high seas does affect international law and the approach of other countries," lawyer George Newhouse told reporters.
"It would be highly unusual for the United Nations to intervene in a High Court case in Australia," he said. "It shows the high level of concern internationally over Australia's treatment of vulnerable men, women and children."
The Australian office of the UNHCR confirmed it had a deadline of Sept. 2 to join the case but had not yet made a decision. In 2006, it joined a case in an Australian court about temporary protection visas for refugees.
The HRC, which is holding an inquiry into the wellbeing of children in Australia's immigration detention centres, said it had sought leave to join the case but did not comment further.
The group of Tamils set off from India on June 13 and was intercepted in Australian waters 16 days later. Legal action was originally launched to prevent them being returned to Sri Lanka.
The case was revised after the government brought the group to Australia and then moved them to the tiny South Pacific island nation of Nauru when they declined to speak with Indian officials about their refugee claims.
Australian Immigration Minister Scott Morrison has said he is confident they are economic migrants and therefore not entitled to asylum.
Newhouse said it was important to have legal clarification about any limits to the government's powers on the high seas because there were untested questions about turning back boats that could have wider implications.
Australia already has offshore camps on Nauru and Papua New Guinea where it processes asylum seekers who arrive by boat. In April, it also agreed to a deal with Cambodia to take in people intercepted at sea.
The number of asylum seekers reaching Australia pales in comparison with other countries but it is a polarising political issue. Conservative Prime Minister Tony Abbott campaigned heavily on the issue before winning elections last year.
The government earlier this week announced plans to release scores of children from detention centres, following criticism from human rights advocates that detaining minors is detrimental to their mental and physical health. (Editing by Paul Tait)
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