When Gus Kuster finished a one-year prison sentence in Australia, he anticipated rebuilding his life there, in the only country he has ever known. Instead, as a noncitizen and stateless person, he spent the following five years being shuttled between grim immigration detention centers, with seemingly no release date in sight.

Dozens of other people, none of them Australian citizens, have been subjected to the same experience. Some, like Mr. Kuster, had served time for minor crimes, others had been found guilty of serious crimes like murder, and a handful had no criminal background at all.

Australia has been criticized for years internationally for its harsh treatment toward asylum seekers, many of whom were housed in the country’s infamous offshore detention centers, where a few dozen people still remain. But hundreds more are still held indefinitely in similar institutions onshore. Until very recently, that included people who were once given a shot at life in Australia, then had that opportunity snatched away after they committed crimes.

Last month, many of these indefinite detentions came to an abrupt end. A detainee successfully challenged the two-decade precedent in Australia’s highest court, and in the ensuing weeks, more than 150 people have been freed. Just as many cases are under review.