When peace activists used a sickle to puncture a plastic radar cover at Blenheim's Waihopai spy base, they temporarily deflated a key link in an organisation that is a global eavesdropper.

Both Waihopai and the Tangimoana radio listening post near Palmerston North have been identified as key players in the United States-led Echelon spy programme.

Though they are run by the Government Communications Security Bureau (GCSB), the bulk of the bases' intelligence is believed to be fed to the US and the other Echelon member nations: Canada, Australia and Britain.

Echelon began life during the Cold War, but has been modified to search for evidence of terrorist plots, drug deal plans and diplomatic intelligence.

However, it has also been accused of carrying out industrial and economic espionage for its member nations.

Waihopai was established in 1989, initially boasting one 18m antenna. Two more were added later. The station listens primarily to Intelsat satellites in orbit above the Pacific Ocean.

The GCSB describes the base's function as "to intercept transmissions from communications satellites and to decrypt and process the signals".

The bureau's radio listening station at Tangimoana is a bunker-like building among the sand dunes of the Manawatu coast, north of Foxton.

It is believed to spy on high-frequency radio communications from ships and land-based telephones.

Though Tangimoana's importance is believed to have lessened, its workings are still cloaked in secrecy.

One nearby resident said the only evidence of a spy station was "a large mailbox and lots of cars arriving in the morning and leaving at night".

Tangimoana and Waihopai stations run "dictionary" software that listens for words pre-programmed by the various spy agencies of the member countries.

Peace activist Nicky Hager first exposed the inner workings of the Echelon programme in his 1996 book Secret Power: New Zealand's role in the international spy network.

"The Echelon system has created an awesome spying capacity for the United States, allowing it to monitor continuously most of the world's communications ...

"Since the Echelon system was extended to cover New Zealand in the late 1980s, the GCSB's Waihopai and Tangimoana stations can be seen as elements of a United States system and as serving that system. The GCSB stations provide some information for New Zealand Government agencies, but the primary logic of these stations is as parts of the global network."

Headquarters: Wellington.
Staff: 314 fulltime equivalents (June 2009).
Director: Sir Bruce Ferguson.
Budget: $49.36m (June 2009).
Bases: Waihopai (Blenheim), Tangimoana (Manawatu.