In a post published on The Intercept website on Monday afternoon, Mr Snowden - a former analyst at the US National Security Agency (NSA) - says he came across the communications of New Zealanders in his work.
"If you live in New Zealand, you are being watched," he wrote.
Prime Minister John Key's claim that there never has been any mass surveillance by the Government Communications Security Bureau was false, Mr Snowden said.
"At the NSA I routinely came across the communications of New Zealanders in my work with a mass surveillance tool we share with GCSB, called XKEYSCORE.
"It allows total, granular access to the database of communications collected in the course of mass surveillance. It is not limited to or even used largely for the purposes of cybersecurity, as has been claimed, but is instead used primarily for reading individuals' private email, text messages, and internet traffic," Mr Snowden said.
"I know this because it was my full-time job in Hawaii, where I worked every day in an NSA facility with a top secret clearance."
Mr Key says there has not been any mass surveillance of New Zealanders.
American journalist Glenn Greenwald is expected to release more details about the GCSB at Internet Party founder Kim Dotcom's "moment of truth" event in Auckland on Monday night.
Livestream the event below:
