New Zealand's legal system will have failed if the US government succeeds in its bid to quash the court-ordered release of the evidence it holds relating to internet tycoon Kim Dotcom's extradition, the Court of Appeal has heard.

Dotcom, who founded file-sharing website Megaupload, and his wife, Mona, were at the Court of Appeal in Wellington on Thursday as the US appealed a District Court ruling that it must disclose its internet piracy case against him so he gets a fair hearing in extradition proceedings next year.

The initial ruling, from Auckland District Court Judge David Harvey, was previously upheld by High Court Justice Helen Winkelmann, who dismissed a US application for a judicial review.

Dotcom's lawyer Paul Davison told the court his client needs access to information relevant to his extradition for natural justice to be served.

"It would be a failure of our legal system ... for one adversary to be fully-armed and another to be deprived of access to information," Mr Davison said.

John Pike, acting for the US, disputed whether the District Court had the jurisdiction to order disclosure in an extradition case.

He cited Canadian precedents, as there was no New Zealand case law to guide the court, arguing the District Court "cannot order disclosure of the foreign requesting state's evidence".

Speaking outside court after the hearing, Dotcom told reporters New Zealand's judicial system had shown it was independent and fair.

"The only thing I'm nervous about is the ongoing delaying tactics by the United States utilising Crown law to appeal every time we win in court and just dragging things out," he said.

"If it was up to them, we would have no money for our legal defence, no evidence to defend ourselves, we would not even have our own computers back to show our own evidence."

The Court of Appeal has reserved its decision.