Newly declassified documents reveal details of New Zealand's part in a top-secret intelligence sharing agreement with the United States and Britain.
The files show that New Zealand and Australia joined what was then called the UK-USA Agreement in 1956, 10 years after the two major powers first reached a deal. Canada joined in 1948.
During the Second World War, Britain and America had co-operated closely on so called "signals intelligence", or intercepted communications.
After the war, they decided to institutionalise that co-operation and establish it in the new context of the emerging Cold War with the Soviet Union.
Until a few years ago, the US and Britain did not even acknowledge the existence of the agreement, which remains the basis for sharing intelligence between all five countries.
Thousands of newly released files reveal private conversations and correspondence between Soviet citizens, military personnel, Communist Party officials and religious leaders in the period from 1946 to 1949.
The intelligence alliance still operates today, and since the incorporation of Australia, Canada and New Zealand, it has been known as the "Five Eyes".
The British National Archives principal records specialist, Ed Hampshire, says other files that show what material was gathered under the agreement will be a treasure trove for historians.