The collaboration began during World War II when the allies were trying to crack German and Japanese naval codes and has endured for more than 70 years. The alliance helps avoid duplication in some instances and allows for greater penetration in others. The five nations have agreed not to spy on each other, and in many outposts around the world, Five Eyes agencies work side by side, allowing for information to be shared quickly. But Richard Aldrich, who spent a decade researching a book on British surveillance, said some Five Eyes nations have spied on each other, violating their own rules. The five countries "generally know what's in each other's underwear drawers so you don't need to spy, but occasionally there will be issues when they don't agree" – and when that happens they snoop, Aldrich said. In Five Eyes, the US boasts the most advanced technical abilities and the biggest budget. Britain is a leader in traditional spying, thanks in part to its reach into countries that were once part of the British Empire. Australia has excelled in gathering regional signals and intelligence, providing a window into the growing might of Asia. Canadians, Australians and New Zealanders can sometimes prove useful spies because they don't come under the same scrutiny as their British and American counterparts. "The United States doesn't share information," said Bob Ayers, a former CIA officer, "without an expectation of getting something in return."

Britain is home to one of the world's largest eavesdropping centres, located about 300 kilometres north-west of London at Menwith Hill. It's run by the NSA but hundreds of British employees work there, including analysts from Britain's eavesdropping agency, the Government Communications Headquarters – or GCHQ. Australia is home to Pine Gap, a sprawling satellite tracking station located in the remote centre of the country, where NSA officials work side-by-side with scores of locals. The US also posts three or four analysts at a time in New Zealand, home to the small Waihopai and Tangimoana spy stations. The intelligence-sharing relationship enabled American and British security and law enforcement officials to thwart a major terror attack in 2006 – the trans-Atlantic liquid bomb plot to blow up some 10 airliners. The collaboration, sometimes called ECHELON, takes place within strict parameters. Two US intelligence officials, who spoke on condition of anonymity because they weren't authorised to speak about the program to the media, said only US intelligence officers can directly access their own vast database. A Five Eyes ally can ask to cross-check, say, a suspicious phone number it has independently collected to see if there is any link to the US, the officials said. But the ally must first show the request is being made in response to a potential threat to Western interests. Ferguson said that in New Zealand, co-operation with the US improved markedly after the September 11, 2001, terrorist attacks. Still, he said, his agency was kept on a need-to-know basis. He said he never knew what information was being provided to other Five Eyes nations, and none of the countries would have shared all their intelligence anyway.

Ferguson said a small country such as New Zealand benefited by a ratio of about five-to-one in the information it received compared to what it provided. He said that as chief of the defence force, a role he held before taking over the spy agency in 2006, he could never have sent troops to Afghanistan without the on-the-ground intelligence provided by the US and other allies. He said New Zealand continues to rely on Five Eyes information for most of its overseas deployments, from peacekeeping to humanitarian efforts. The intelligence is vital, he added, for thwarting potential cyber threats. In Australia, prosecutors in 2009 used evidence from a US informant who had been at a terrorist training camp in Pakistan to help convict one of nine Muslim extremists found guilty of planning to bomb an unspecified Sydney target. "Intelligence sharing between countries is critical to identifying and preventing terrorism and other transnational security threats," said the Australian Security Intelligence Organisation, or ASIO. Canada's Department of National Defence had a similar response, saying it "takes an active role in building relationships with allies. Collaborating with the personnel of the Five Eyes community in support of mutual defence and security issues is part of this relationship building." Both agencies declined requests to provide more specific information. In the decades since World War II, the allies have formed various other intelligence allegiances, although few as comprehensive or deep as Five Eyes. While the Snowden revelations will test the relationship, it has survived tests in the past.

New Zealand has long asserted an independent foreign policy by banning nuclear ships, and some are now calling for the country to go further and opt out of Five Eyes. Russel Norman, co-leader of New Zealand's Green Party, is one of many people calling for a public review of the relationship. "I want to live in a free society, not a total surveillance state," he said. "The old Anglo-American gang of five no longer runs the world." But John Blaxland, a senior fellow at the Australian National University's Strategic and Defence Studies Centre, said politicians Down Under have often criticised the security relationship until they've gotten into power and been briefed on its benefits. Then, he said, they tend to go silent. "The perception is that the advantages are so great, they'd be crazy to give it up," he said.

AP Follow IT Pro on Twitter