The Crown Law Office in New Zealand has filed papers opposing a court ruling that would allow Kim Dotcom's legal team to view the evidence gathered against him.
Mr Dotcom's lawyers had asked to see the evidence to be used against their client.
But Crown lawyers, representing US authorities, say an application for a judicial review was filed on Thursday in North Shore District Court.
On Tuesday, Judge David Harvey ruled Mr Dotcom's lawyers should have access to some material relating to an extradition hearing that is set down for August.
The United States is seeking to extradite the Megaupload founder to face charges of internet piracy, money laundering and fraud.
US lawyer Ira Rothken says he has moved to have the case thrown out and the company's assets unfrozen.
Megaupload was indicted in a 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 United States in early March.