The New Zealand judge due to hear Kim Dotcom's extradition case has stepped down from the affair after he referred to the US as "the enemy" during an internet conference discussion on copyright laws.

Judge David Harvey, an internet law expert, was due to hear German-born Dotcom's extradition case in March next year, where the US wants to try him on racketeering, copyright and money laundering charges over his file-sharing Megaupload website.

However, at last week's NetHui conference, Judge Harvey cited a tweet by journalist Russell Brown: "We have met the enemy and he is [the] US."

That referred to a quote by US cartoonist Walt Kelly - "We have met the enemy and he is us" - to an argument over international copyright.

The comment attracted media attention.

District Courts' chief judge Jan-Marie Doogue said on Wednesday Judge Harvey had made the decision to step down from hearing the case and Judge Nevin Dawson will now hear the case, Radio New Zealand reports.

The New Zealand government is negotiating a Pacific Rim free trade agreement with a number of countries, including the US.

Critics say the Trans-Pacific Partnership agreement will mean US companies will be able to dictate New Zealand laws in areas around copyright.