The National Security Agency (NSA) illegally used technology to spy on Megaupload founder Kim Dotcom, according to new documents from New Zealand's Government Communications Security Bureau (GCSB).

The New Zealand Herald first reported that the GCSB told the nation's high court that it ceased all surveillance of Dotcom in early 2012, but that "limited" amounts of communications from Dotcom were later intercepted by its technology without the bureau's knowledge.

Dotcom was surveilled by the NSA and the GCSB in a joint intelligence operation named Operation Debut. According to the Herald, that surveillance was scheduled to end in January 2012, but the United States continued to use New Zealand's technology.

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According to court documents obtained by the Herald, "Limited interception of some communications continued beyond the detasking date without the knowledge of GCSB staff."

The court papers don't explain how the NSA was able to use the GCSB's spying technology without the bureau's knowledge. According to the Herald, "The GCSB documents do contain an admission of NSA involvement, although it was not made outright."

Dotcom is facing charges of copyright infringement and money laundering related to Megaupload, a file-sharing website shut down in 2012. He is currently fighting U.S. attempts to extradite him from New Zealand.

In an interview with the Herald, he blasted the NSA for illegally using the GCSB's spying infrastructure and warned New Zealanders about the power the U.S. wields over their data.

"New Zealanders must know how much power a foreign state holds over their private information," Dotcom said.

"The NSA has unrestricted access to GCSB surveillance systems. In fact, most of the technology the GCSB uses was supplied by the NSA."

Dotcom said his legal team would take further action based on the news.

"If the GCSB was aiding and abetting the NSA to spy directly on New Zealanders then the seriousness of the situation has changed dramatically and a truly independent inquiry and a new criminal investigation will be unavoidable."