"Guantanamo Bay is an abhorrent aberration of everything that my country ought to be about," she told reporters in Sydney yesterday, the Australian reports.
"I think it's because we do execute people. It just so smacks of empire."
Sr Prejean also said the release of Australian terror suspect David Hicks should be a priority.
"When you have a human life caught in that impossible situation where you don't have recourse to any of the means of justice, you want to get him out of there," she said.
Sr Prejean offered spiritual guidance to the man facing execution and was with him when he was taken to the electric chair.
"This was my journey ... like most people I had never seen someone killed before, but I wasn't going to let him go through that alone," said Sr Prejean, who was in Sydney on Monday to promote the opera.
"It takes you through it, the troubles and the crime. You want to ask how I feel about that (it being made into an opera)? I am delighted."
Sr Prejean said attitudes to capital punishment in the US were changing. For the first time, recent polls had supported life sentences slightly over the death penalty.
Asked about the proposed joint appeal to Indonesia's Constitutional Court on behalf of the Bali bombers and three of the Bali Nine, Sr Prejean said: "Once you lump things together, especially where death is the penalty, there's a blurring that begins to happen, and life gets even more cheapened."
The opera of Dead Man Walking opens in Sydney in September.
The New Zealand-born baritone Teddy Tahu Rhodes will reprise the role of convicted killer Joseph de Rocher, which he sang in an Adelaide production of the same opera in 2003.
In a surprise move, broadcaster Alan Jones has a role in the opera as a lawyer.
"Obviously Alan is completely against the death penalty," producer Nicole Alexander told reporters.
Death-row nun lashes Guantanamo jail (The Australian, 27/3/07)
Alan Jones to appear in death row opera (Ninemsn, 26/3/07)
LINKS (not necessarily endorsed by Church Resources)
Helen Prejean website
Helen Prejean (Wikipedia)
Death of Innocents website
Death row movie nun set to visit Australia (CathNews, 14/2/05)
Sr Prejean leads vigil as Texas executes killer (CathNews, 23/3/00)
27 Mar 2007