Details of the construction of the top secret spy base at Tangimoana were filed with Archives New Zealand with no security rating, allowing the contents of the file to be copied.
The Archives NZ file which contains the details has now had access restricted for 100 years after the Herald told the Government Communications Security Bureau it had a copy.
The construction file included details about the security system protecting the spy base.
The Herald came across the file and other GCSB information while reviewing files about the security services held by Archives NZ, the national repository for public records.
Built in the early 1980s, Tangimoana was a critical signals link for Western powers during the Cold War.
While the building report and details are 35 years old, the emergence of closely held GCSB secrets in a publicly accessible archive goes against every security instruction under which the GCSB operates.
There are few records available about the bureau in Archives NZ and almost all are restricted for at least 70 years.
Air Marshal Sir Bruce Ferguson - former Chief of Defence Force and GCSB director - said the bureau's security procedures meant it had to consider the possibility an adversary could jigsaw information together and expose secrets.
He said "an innocent piece of information, even a little bit" has the potential to create a potentially fatal security risk.
"The want of people to know is great. But if that compromises even one life? And that could quite easily happen."
A GCSB spokeswoman said there was concern the documents could be matched with other information and create a security risk.
The file had been sent to Archives NZ by the Ministry of Works, the government department once charged with overseeing Crown construction jobs. It appears this was how it wound up bypassing security screening - a similar glitch once struck the GCSB when a top secret report was found among the files of former prime minister David Lange.
The GCSB spokeswoman said: "We have requested that Archives New Zealand restrict access to the documents. This means the GCSB will consider whether or not to release on a case by case basis. This is in line with access to documents of a similar nature that archives hold."
An Archives NZ spokeswoman said access to the files had been changed after a review by the agencies involved.
It meant the file now had restricted access for 100 years from the date it was closed "in order to protect national security and international relations, and preserve personal privacy".
She said members of the public could apply to the GCSB to review access to the file.
The plan to build the spy base was called "Project Acorn" and was overseen by the Ministry of Defence, which set up the GCSB in September 1977. The building project was conceived two years later with building completed by 1981.
The file is classified "restricted", placing it between "sensitive" and "confidential" according to the GCSB's classification system.
The GCSB says release of "restricted" material has the potential to "adversely affect diplomatic relations", "hinder security of NZ forces or friendly forces", "adversely affect internal stability of NZ or friendly countries" and "adversely affect economic well-being of NZ or friendly countries".
The file contains security system details including the make and model of an "intruder alarm" fitted to the fence of the Tangimoana base. It also details the type of electronic systems used to secure the doors inside the base, listing the access points that needed protection.
The project saw the Defence Force send in military engineers to build a road and prepare the construction site. Keeping military oversight was essential and a warrant officer was assigned to be there daily.
The file also appears to make reference to the then-secret Five Eyes grouping of nations made up of NZ, Australia, Canada, the United States and United Kingdom. One memo says the design report "has been passed to the sponsor who has been consulted during the development of the sketch plans and report".
The details in the report include technical aspects designed to protect the computer and radio equipment which would later be installed. The report gives precise descriptions of the layout of the building and includes details such as where power sources were located and where cables would run.
The Project Acorn contract was won by a local Palmerston North firm R Okey Ltd with an estimated build cost of $1.4 million. The Clerk of Works report into the project says the small firm did an excellent job with Ron Okey's demands for high standards being passed on to subcontractors.
Okey told the Herald the project was run under strict secrecy orders from the Ministry of Defence. He said his workers ran foul of the security rules when they got T-shirts printed with Chip 'n' Dale cartoon characters next to a pile of acorns - the link being "secret squirrels" and Project Acorn.
"We got told off in no uncertain terms."
For all the secrecy, Okey was left amazed that the plans for the site were never collected.
"They were so uptight about security but they never collected the drawings. I thought it was a bit funny at the time."