A New Zealand High Court ruled on Friday that authorities who raided Megaupload owner Kim Dotcom's mansion earlier this year wielded deficient search warrants, stripping the whole operation conducted by NZ police teams and agents from the U.S. Federal Bureau of Investigation (FBI) of any legal protection.

Justice Helen Winkelmann's ruling favoured Mr Dotcom's earlier contention that NZ government and U.S. operatives who swooped down on his rented Auckland residence acted illegally.

Justice Winkellmann cited in his her decision that warrants lacking specific scopes or purpose are rendered illegal in New Zealand laws.

She added by basing their actions on general warrants that overlooked specific offences or items that authorities need to confiscate during the operation, officers at the scene technically trespassed into a private residence and exceeded the roles mandated to them by laws.

The warrant also failed to lay down specific explanations if the money laundering and copyright infringement charges being brought against the German-born Dotcom were actually in contravention of existing laws in the United States, Justice Winkellmann said.

It was her observation, Justice Winkellmann said, that the joint operation amounted to unreasonable search and seizure acts on Mr Dotcom's private residence.

What had transpired caused "confusion to the subjects of the searches," who were compelled to believe that the warrants were in fact "authorising a search for evidence of offences as defined by New Zealand law," The WA Today reported the court decision as saying.

Justice Winkellmann also declared as illegal the alleged copying of data on hard disks that authorities seized from Dotcom, which his lawyers said were cloned by FBI analysts shortly after the March raid at his home sent to the United States for further probe.

What the FBI did was unlawful, the High Court said, as it ordered the return of copied data still under the custody of NZ authorities.

Justice Winkellmann also ruled that no more copied hard disks should be shipped out of New Zealand.

In a statement, Ira Rothken, Mr Dotcom's U.S.-based lawyer called the court ruling as a major setback on the cases being built up by America against his client.

"In terms of egregious behaviour, this is at the high end of the scale of egregious, wrongful intrusion on privacy," Mr Rothken told the Australian Associated Press (AAP) on Friday.

"One would think, with such a large case, that (U.S. authorities) would have a higher standard of care in how they conducted themselves," he added.

The U.S. shutdown Mr Dotcom's Megaupload on accusations that his file-sharing site had facilitated the illegal distribution of music, movie and software products by American companies, leasing to losses of about $400 million.

The flamboyant former hackers now faces extradition request from the U.S. government for him to be charged with racketeering, copyright and money laundering charges before a U.S. court.