One of Kim Dotcom's Megaupload mates has been arrested in the US - sparking speculation he's about to cut a deal with the FBI and testify against him.

Court documents obtained by the Herald show Andrus Nomm, who'd been awaiting an extradition hearing in The Netherlands, has been arrested.

He is the first of seven men the US accuses of running the defunct Megaupload as a global piracy enterprise.

Four, including Kim Dotcom, are here awaiting an extradition hearing in June, one is in Slovakia and another is in Germany.