Lawyers for internet tycoon Kim Dotcom say the FBI illegally took copies of material from his computers out of the country.

A judicial review has been under way at the Auckland High Court over the legality of the search warrant police used to seize items from the alleged internet pirate's Coatesville mansion.

Mr Dotcom's lawyers say not only did police take items that were not relevant to the charges he faced, but computer data that should have remained in the country was sent offshore to United States authorities.

Chief High Court Judge Helen Winkelmann had asked the Crown, which is acting on behalf of US authorities, to explain why the material was sent.

Defence lawyer Willie Akel told the court 17 clones, or copies, of some computer data was sent to the FBI, despite assurances it would not be.

The Mutual Assistance and Criminal Matters Act, he says, made it illegal for the copies to be sent.

Mr Akel says that throughout several court hearings since January, "no documents in any way refer to the items being offshore".

However, the Crown, on behalf of the US authorities, says the defence knew the copies would be sent to America.

Crown lawyer John Pike says an email from March between the Crown Law Office and Mr Dotcom's Queen's counsel, Paul Davison, shows the defence knew the copies were being sent.

But the defence dispute this, saying that email is not a reliable source because it is ambiguous and gives no dates.

Mr Dotcom is due to face an extradition hearing in August being sought by the US.

It is alleged he was involved in a "mega conspiracy" through the operation of his Megapupload website to distribute pirated material.

Justice Winkelmann has reserved her decision.