Newly declassified files from the Security Intelligence Service show secret police spied on future Labour Prime Ministers in the 1920s who were suspected of having Communist sympathies.
The SIS has released thousands of pages of secret files on Michael Joseph Savage, Peter Fraser and Walter Nash from a spying operation on the Labour Party in the 1920s and 30s. The spies were amassing dirt files on the men who would later become their political masters.
Their personal files were destroyed, but other Special Branch and World War II Security Intelligence Bureau files recorded between 1920 and 1945 have been transferred to Archives New Zealand.
A personal file was located on Auckland's longest-serving mayor Sir Dove-Myer Robinson, a four-times married Communist Party member, whose reign as mayor spanned four decades and who was the subject of a 2012 biography.
The file was released to his daughter, Heather Levack, who said the family wanted to keep the contents to themselves. "Of course it contained sensitive stuff about my dad," she said. "But we've had a gutsful of all the headlines and we'd prefer to keep it to ourselves."
Political commentator Chris Trotter said the files provided a fascinating historical insight.
"In some parts of the world that kind of police activity is just buried forever."
He said most New Zealanders would be surprised about mass surveillance on politicians.
Walter Nash, Prime Minister from 1957-60, was spied on at Irish Republican meetings and for importing "seditious literature" from Australia.
His great-grandson Stuart Nash, Labour Party candidate for Napier in the upcoming election, said there had been anti-left wing paranoia because of the "spectre of Stalin".
"I would doubt they found any direct links to [Josef] Stalin in there."
SIS files on Bill Sutch, who was accused and acquitted of espionage in 1974, were released in 2008.
The Special Branch was set up in 1920 "to investigate and report on revolutionary matters in New Zealand", its main target being the Communist Party of New Zealand.
Many personal files were destroyed under the Privacy Act 1993, which stated "an agency that holds personal information shall not keep that information for longer than is required for the purposes for which the information may lawfully be used". Since 2005, legislation has changed so only the chief archivist can allow records to be destroyed.
The files, now available for public viewing at Archives New Zealand in Wellington, also show the police special branch targeted Christian groups, including the Christian Pacifist Society during World War II.
In 1942, Jehovah's Witnesses were declared a subversive organisation and two members were fined for handing out Christian pamphlets. The following year, the order was reversed.