Dragan Vasiljkovic, also known as Daniel Snedden, was arrested last week in northern NSW after 43 days on the run.
The 55-year-old is wanted in Croatia, where he is accused of killing Croatian citizens and ordering others to murder when he was a paramilitary commander with the Krajina Serbs fighting against Croatia from 1991 to 1995.
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On March 30 the High Court determined he was eligible for surrender to Croatia, but he could not be found.
The Australian Federal Police, with crucial cooperation from Dutch police, eventually tracked him to a property near Coffs Harbour.
He is now being held in Sydney's Silverwater Jail.
'Secondhand statements'
The High Court on Wednesday released the reasons for its March 30 decision.
It said Vasiljkovic had appealed extradition on the basis that Croatia would punish him for his political beliefs, as courts there used a "mitigating factor" when sentencing people who had fought on their side.
Vasiljkovic's case relied heavily on two reports from the Organisation for Security and Cooperation in Europe.
One related to a March 2006 case on incidents in the Lora military prison in Split, while the other related to domestic war crime trials in 2005.
But the High Court said the evidence not only came too late for proper consideration, but also that the reports were not authoritative on sentencing, "let alone in relation to the extremely vague, indirect and secondhand statements" relied on by Vasiljkovic.
"The respondent's endeavour to prove that conclusion came too late, and the materials it relied on to support that conclusion were too feeble," it said.
Home Affairs Minister Brendan O'Connor will consider Snedden's case and plans to make a decision on his extradition as soon as possible.
