The Government Communications Security Bureau is carrying out widespread surveillance of New Zealanders which raises serious questions about the truthfulness of the government, United States journalist Glenn Greenwald says.

Prime Minister John Key disputes the claims.

Greenwald is in New Zealand for Internet Party founder Kim Dotcom’s “moment of truth” announcement on Monday.

He told TV3’s The Nation he had been looking through US National Security Agency (NSA) documents obtained by Edward Snowden which revealed the extent of New Zealand’s surveillance activities as one of the Five Eyes countries, with the US, UK, Canada and Australia.

Greenwald says the documents contradict a statement by the government that New Zealand doesn’t engage in mass surveillance or target New Zealanders unless they are involved in activities such as terrorism or cyber crimes.

The government changed the law last year allowing the GCSB to spy on New Zealanders on behalf of the SIS and police, following revelations the GCSB may have illegally spied on New Zealanders.

Greenwald said he had spent several months looking into New Zealand’s involvement in spying to investigate the truth of statements.

“I can tell you there are serious questions about whether the current government was at all truthful with its citizens in connection with that bill.”

Mr Key said when the law was changed the GCSB had not been involved in the “wholesale collection of metadata about New Zealanders”.

Greenwald told the program: “the government does engage in extraordinary amounts of analysis of metadata – meaning whose talking to who, for how long, where they are when they speak – on a massive indiscriminate scale not just internationally but of New Zealanders as well.”

Mr Key said Greenwald, “Dotcom’s little henchman”, was wrong.

“There is no mass surveillance of New Zealanders by GCSB and there never has been mass surveillance of New Zealanders by GCSB,” he told reporters on Saturday.

“One of the problems is when you hack into people’s information and you steal it sometimes you get part of the information but not all of the information.”

Greenwald said New Zealand spent an extraordinary amount of resources on electronic surveillance for a country of its size.

“Every single thing that the NSA does…involves NZ directly.”

New Zealand spied on a variety of countries, hostile and allies, on behalf of the US and UK, he said.