WELLINGTON, New Zealand—Ex-U.S. intelligence employee Edward Snowden accused New Zealand Prime Minister John Key of deceiving the public about the mass surveillance of the nation's citizens by the country's domestic spy agency.

In an article written for the Intercept website, Mr. Snowden said that in his former role, he routinely came across the communications of New Zealanders that had been captured by a mass-surveillance tool he claimed was used by the Government Communications Security Bureau, or GCSB.

"Any statement that mass surveillance is not performed in New Zealand, or that the internet communications are not comprehensively intercepted and monitored, or that this is not intentionally and actively abetted by the GCSB is categorically false," Mr. Snowden wrote.

Without referring to the specific allegations, Mr. Key said in a statement Monday that claims circulating about the spy agency were incorrect.

The Intercept is a website affiliated with U.S. journalist Glenn Greenwald, who has worked closely with Mr. Snowden in the past to help leak the activities of U.S. spy agencies. Mr. Greenwald, who is in New Zealand, further expanded on the allegations against the country's spy agency at an event Monday hosted by Internet entrepreneur Kim Dotcom, whose Internet Mana Party is competing in a general election to be held Saturday.