US authorities have been ordered to hand over evidence against Megaupload's founder and other employees of the file-sharing site.
The US wants to take charge of the men from New Zealand to put them on trial for alleged copyright infringement, money laundering, racketeering and other offences.
Defence lawyers want the information to help them challenge the extradition request.
The US had previously objected.
However, New Zealand District Court's Judge David Harvey said local laws required extradition hearings to be "properly informed" adding that the case would be "one-sided" and merely an "administrative" affair were the material not to be shared.
Megaupload was shut down earlier this year in what the US Justice Department described as one the "largest criminal copyright cases" ever brought by the country.
In an 81-page ruling, published online by the news site Torrentfreak , the judge noted that much of the evidence against founder Kim Dotcom and his associates was "contained on their computer equipment which was seized as a result of the activities of January 2012" - a reference to evidence gathered from search warrants executed in nine countries that month.
The judge concluded that the material should be disclosed by 19 June so the court could consider the plausibility of the criminal copyright charge which he said was a "keystone" to the other accusations.
Ars Technica reported that the judge had also issued a separate ruling saying Mr Dotcom was not a flight risk and was therefore freed of the requirement to be electronically monitored.
The move means Mr Dotcom can return to his rented mansion. The property had previously been judged to be unsuitable for the monitoring equipment.