Prime Minister John Key says it was not in the public interest for New Zealanders to know the GCSB had been considering plans to build a cyber protection network but he has no option but to release the information now.
Mr Key says he will release documents to back up his argument that New Zealand never got close to carrying out mass surveillance, to protect his reputation against attacks by American journalist Glenn Greenwald.
Listen to John Key on Morning Report
Listen to Glenn Greenwald on Morning Report
Mr Greenwald will release information on the Government Communications and Security Bureau's activities at an event hosted by the Internet Party founder Kim Dotcom in Auckland tonight.
Mr Key says Mr Greenwald's accusations refer to a proposal about collecting information to prevent cyber-attacks which the agency was working and which was discussed with the United States and other countries.
He said the idea was never put into action, because he decided not to go ahead with it.
"Greenwald's going to turn up tonight as part of a Dotcom sound and light show five days before an election trying to influence New Zealanders.
"He's made claims on television ... that are deliberately attempting to damage my reputation and tell New Zealanders that I'm doing something that I'm not and I've got no option but to protect myself."
Mr Key told Radio New Zealand's Morning Report programme he will declassify spy documents to back his arguments, but only after Mr Greenwald releases his evidence.