In the latest of several appeals an Australian court found that alternatives including prosecution at home had not been explored sufficiently. The ruling saw an extradition order made in October 2009 overturned.
Justice McKerracher of the Federal Court ruled that "the more humane solution" of trying the suspect in Australia had been dismissed without justification, and that it would be oppressive to extradite someone of advanced age and poor health.
The case against Mr Zentai was first brought by the Simon Wiesenthal Centre. Mr Zentai is one of their 10 most-wanted suspects.
The Australian government has not said whether it will mount a further challenge. But Dr Zuroff said he was disappointed and will continue to push for an appeal.
He said: “The whole issue of his age is totally irrelevant because there's never been a single country, including Australia, which has ever limited prosecution based on age.
“The fact that many years passed without him being brought to justice is totally irrelevant, the passage of time in no way diminishes the guilt of the killers.”
