A German millionaire should be sent to the US to face internet piracy charges, a New Zealand judge ruled on Wednesday.

Kim Dotcom, who headed the Megaupload online file-sharing site, and his co-accused Mathias Ortmann, Bram van der Kolk and Finn Batato, were eligible to be extradited, Xinhua cited the judge as saying at an Auckland district court.

New Zealand Police arrested Dotcom in Auckland on January 2012 at the request of the US Justice Department and the Federal Bureau of Investigation (FBI) on charges including copyright infringement, wire fraud, money laundering and racketeering.

Radio New Zealand reported that Judge Nevin Dawson ruled there was an "overwhelming preponderance" of evidence against the men that the now defunct Megaupload site had aided and abetted users who uploaded pirated files by paying them rewards.

Under New Zealand's Extradition Act, Justice Minister Amy Adams must now decide whether to confirm the court's decision and go ahead with the extraditions.

But the four men could still appeal the court decision and Dotcom's US lawyer, Ira Rothken, had tweeted that they intended to do so, said the report.

During the 10-week court hearing that ended last month, the men's lawyers argued Megaupload was created simply to allow people to store and share large files.

But the FBI claimed the site wilfully breached copyright on a mass scale by hosting illegally-created movie, music and software files.

The US authorities claimed Megaupload's senior employees aided and encouraged piracy by paying cash rewards to uploaders whose files proved popular, driving traffic and paying subscribers to the site.

The dramatic arrest of Dotcom by armed police at the Auckland mansion almost four years ago triggered a chain of events that cost at least one Cabinet minister his job.

During various court proceedings, it was revealed that New Zealand's government security agencies had been illegally spying on New Zealand citizens and residents.

Dotcom also established and funded the internet party to fight last year's general election and derail the ruling centre-right government, but it failed to win a single seat in Parliament.

During the election campaign, Dotcom organised a public meeting with video links to the US National Security Agency whistle-blower Edward Snowden and Wikileaks founder Julian Assange to discuss claims that New Zealand spy agencies were carrying out mass surveillance on behalf of the US.

Before the election cabinet minister John Banks was forced to resign his post over allegations of failing to properly record campaign donations from Dotcom during his time as Auckland mayor.

Banks was forced out of the parliament after he was convicted of the charges, but he was later cleared on appeal.