The police raids on internet mogul Kim Dotcom's mansion were legal, the Supreme Court has ruled.

The court has upheld a Court of Appeal decision that while the search warrants could have been more precisely written they were legal.

The warrants, executed by police on the properties of Dotcom and his associate Bram van der Kolk in January 2012, resulted in the seizure of some 135 electronic items.

Dotcom, van der Kolk, Finn Batato and Mathias Ortmann, who appealed against the decision, were ordered to pay costs of $35,000 to the Crown.

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In today's decision, Chief Justice Sian Elias and Justices John McGrath, William Young, Susan Glazebrook and Terence Arnold concluded the warrants warrants were not unreasonably vague and general.

"[But] undoubtedly they could have been drafted rather more precisely."

The United States authorities had charged Dotcom and others with particular offences, including money laundering and criminal breach of copyright, the decision said.

"The criminal activity alleged is extensive and is alleged to have been carried out through what, outwardly, resembled a legitimate large-scale cloud storage facility."

Through the New Zealand police, the United States authorities sought and obtained warrants to search for and seize material, including computers, relevant to that alleged offending, the decision said. The warrants did not purport to authorise the seizure of things that were not relevant to the suspected offences.

"The computers were plainly relevant to the offending alleged, although some of their contents were undoubtedly irrelevant."

The warrants recorded the police belief that there was evidence on the sites to be searched that related to reproduction and distribution of copyrighted works "including but not limited to motion pictures, television programmes, musical recordings, electronic books, images, video games and other computer software", it said.

Communications related to the activities of the "Mega Conspiracy" were also specified, naming "Megaupload, Megavideo and Megastuff Limited", albeit inclusively.

"Moreover, the search warrants were served and executed immediately following the arrests of the appellants, who were given copies of their arrest warrants."

There appeared to be no dispute that the appellants ran their business activities from the properties, which had multiple, sophisticated internet connections, the decision said.

"Finally, those conducting the searches were briefed on what they were searching for.

"Accordingly, we agree with the Court of Appeal that the appellants were reasonably able to understand what the warrants related to and that the police were adequately informed of what they should be looking for."

They dismissed the appeal.

Dotcom's extradition hearing is set for June. He is also expected to try to have some of his assets released by the courts in the near future.