Lawyers for the US government have asked a New Zealand court to overturn earlier rulings requiring the FBI to provide its evidence against Kim Dotcom to his legal team.
The United States wants to extradite the co-founder of the file-sharing site Megaupload on copyright, money laundering and fraud charges.
A district court judge ruled in May that the FBI should hand over copies of its evidence relating to the extradition bid. That ruling was upheld by the High Court at Auckland.
Crown lawyer John Pike, acting for the US Government, told the Court of Appeal in Wellington on Thursday the district court had no power to order such disclosure in an extradition case.
However, Mr Dotcom's lawyer Paul Davison said the disclosure was absolutely necessary so his client could determine whether the evidence relied on by the United States met the standard required in an extradition case.
He said legally extradition hearings must be conducted in the same way as a committal for trial under New Zealand criminal law.
Mr Pike said the district court ruling did not grant Mr Dotcom's lawyers all they were seeking.
However, he said the disclosure ordered was extensive and could involve billions of emails and other items of information that underpin the Dotcom prosecution.
Lawyers for Mr Dotcom say that without access to the FBI's evidence against him, he will not be able to adequately prepare his legal fight against extradition.
The Court of Appeal has reserved its decision.
Dotcom confident of fair hearing
Heading into court on Thursday morning, Mr Dotcom, a German national with New Zealand residency, said he was confident he would get a fair hearing.
"Well, so far the judicial system in New Zealand has been very good and very fair, so I think we will see more of that."
Kim Dotcom says the action being taken by the US is wrong and he is a victim of its bully-boy tactics. He believes it is a political attack on his business and has no legal grounds.
Megaupload was indicted in a US federal court in Virginia on 19 January this year and the site was shut down. Mr Dotcom and three co-accused were arrested in Auckland the following day.
Formal papers requesting their extradition were lodged by the US in March.