The United Nations report also criticized Australia’s practice of detaining children, including infants. In 2009, three children under the age of eight were held with their parents at the Villawood facility. The report found that detention had severely impaired the children’s psychological development. The family was finally released last year, after the Australian Security Intelligence Organization revised its assessment.

In making a case for detention, Australia’s immigration department relies on a security assessment of each prisoner, covering everything from espionage to terrorism and people-smuggling. The burden of proof is not high; detention can be upheld even if the A.S.I.O. deems it relatively unlikely that the person under assessment may commit harm. As the organization is not legally required to disclose evidence, little is known about why specific risk designations are upheld. Many detainees do not know the grounds on which they are being held. Because no court or tribunal can independently test the organization’s claims, it is impossible to know whether the detainees are truly dangerous.

The security organization asserts that the Villawood and Maribyrnong prisoners might commit politically motivated acts of violence. In the case of the Tamil detainees, it alleges prior relationships with the Liberation Tigers of Tamil Eelam, the rebel group that carried out attacks and suicide bombings during the Sri Lankan civil war. But few of the detainees fought in that war, and none is alleged to have harmed civilians or committed terrorist acts. Many were nominally associated with the Tigers because they lived in parts of Sri Lanka that fell under Tamil control. One was a civilian lawyer for the Tamils; another dug ditches to shelter civilians. Australia’s immigration department in 2010 and 2011 extraordinarily granted refugee status to all of the detainees; a designation explicitly denied anyone who has committed past acts of terrorism, or who is believed to pose a serious future risk.

Few Australians, it would seem, are troubled by the plight of the detainees. Certainly no sizeable political constituency has expressed concern, perhaps because “boat people” are generally unpopular. But perhaps the larger problem is that, since reporting inside detention centers is restricted, the refugees have largely remained invisible.

Sustained international pressure is therefore essential. Australia cares greatly about its reputation, particularly with democratic peers like Britain, the European Union, Canada, Japan and Indonesia. Even China, Australia’s largest economic partner — and a country whose own human rights record is hardly unblemished — could be a useful lever. During a bilateral human rights dialogue last month, China rebuked Australia’s treatment of the refugees; Australia is sure to come in for even harsher criticism when it appears in July 2015 before the United Nations Human Rights Council.