Search warrants used by police to raid the home of Megaupload founder Kim Dotcom's have been declared invalid, dealing a major blow to the FBI's ongoing anti-copyright infringement campaign.

The New Zealand High Court ruled that the warrants did not sufficiently outline Dotcom's alleged crimes.

"The warrants did not adequately describe offences to which they related. Indeed they fell well short of that. They were general warrants and as such, are invalid," said High Court judge Helen Winkelmann.

The raid in question occurred in January, with 70 New Zealand police officers raiding Dotcom's mansion. The operation was conducted at the behest of the FBI, which believes Dotcom - also known as Kim Schmitz - is the kingpin of an internet piracy ring.

Prosecutors have since estimated that the ring netted $175m by providing access to illegally copied music, movies and other copyrighted content before being shut down.

Dotcom's lawyers have contested the charges maintaining that the company only offered online storage.

The decision is a blow to the FBI's global anti-copyright campaign. The law enforcement agency is currently seeking the extradition of several alleged copyright infringers including British citizen Richard O'Dwyer.