Megaupload founder Kim Dotcom told the South by Southwest festival Monday that "interesting facts" will emerge in his ongoing fight against extradition to the United States over copyright violation.

Speaking via Skype video link from New Zealand, Dotcom promised "a really cool hearing" in April that is to focus on evidence surrounding the commando-style raid on his home outside Auckland in January 2012.

"There will be interesting facts revealed," added the German-born entrepreneur and one-time teenage hacker who is wanted by the US Justice Department for alleged copyright violation and racketeering.

Without elaborating, Dotcom claimed it would be shown that the prime minister of New Zealand, John Key, misled the country's parliament in relation to his case, which has captivated the online world.

Dotcom reiterated his belief that his case -- which shut down the Megaupload file storage site, causing customers worldwide to lose data they had uploaded -- was politically motivated.

"Get the popcorn ready," said Dotcom, 39, appearing like a cheerful ghostly face against a pitch-black background on a giant projection screen, "because you won't believe what these guys did."

And he predicted victory in the end. "I will never be in a prison in the United States," he said to applause from the audience of several hundred. "I can guarantee you that."

US authorities allege Dotcom's Megaupload and related file-sharing sites netted more than US$175 million and cost copyright owners more than US$500 million by offering pirated copies of movies, TV shows and other content.

Dotcom, a German national who changed his name from Kim Schmitz, faces an extradition hearing in August. Until then, his passport has been seized and he cannot travel out of the country.

Last week an appeal court in New Zealand upheld Dotcom's right to sue New Zealand's foreign intelligence agency for illegally spying on him as part of a US probe into alleged online piracy.

The Court of Appeal refused a government request to overturn a High Court decision in December that said he can seek damages from the Government Communications Security Bureau (GCSB) over his treatment.

The ruling also meant the GCSB would have to disclose to Dotcom's defence some details of information-sharing arrangements it had with foreign agencies, including US authorities, before the Internet tycoon's arrest in January 2012.

It emerged last September that the GCSB spied on Dotcom before armed police raided his Auckland mansion, even though he is a New Zealand resident and should have been off-limits to the agency.

Following the revelation, which prompted an apology from Prime Minister Key, Dotcom applied to include the GCSB in a lawsuit he is planning against New Zealand police alleging wrongful arrest.

Dotcom's lawyers have not detailed how much compensation they want but opposition political parties said the amount could be substantial.