The head of Australia's main intelligence agency on Wednesday revealed the existence of an "A-Team" of foreign spies that for several years has recruited Australian academics, politicians and businesspeople, gaining access to sensitive political, economic and defence information.

In an extremely rare public revelation of counterespionage operations, Mike Burgess, Australia's director-general of security, revealed the existence of the unit, belonging to an unnamed country, in a bid to disrupt ongoing operations.

"Right now there is a particular team in a particular foreign intelligence service with a particular focus on Australia -- we are its priority target," Burgess said in a speech in Canberra.

"We call them 'the A-team' –- the Australia team."

"Many of the people here tonight are almost certainly high-value targets. The team is aggressive and experienced; its tradecraft is good -- but not good enough," he said.

Burgess, who runs the Australian Security Intelligence Organisation (ASIO), said the unit had targeted Australians with access to "privileged information" on social networking sites using "false, anglicised personas" and promising cash rewards.

"The spies pose as consultants, head-hunters, local government officials, academics and think tank researchers, claiming to be from fictional companies such as Data 31," he said.

"If a target takes the bait, the spies try to move the conversation onto an encrypted messaging app. A further step might involve the offer of an overseas trip to meet in person."

He outlined examples of when the approach had been successful, gleaning information from an academic and an unnamed former politician who "sold out their country, party and former colleagues to advance the interests of the foreign regime".

"At one point, the former politician even proposed bringing a prime minister's family member into the spies' orbit," he said.

- Spy vs spy -

Explaining why he had declassified information about the unit, its operations, and the counterespionage measures taken against it, Burgess said he wanted to warn Australians of the risks.

"On just one professional networking site, there are 14,000 Australians publicly boasting about having a security clearance or working in the intelligence community. Some even out themselves as intelligence officers," he said.

Australia is a member of the Five Eyes intelligence-sharing group that includes the United States, Britain, Canada and New Zealand -- making it a juicy target for operatives from countries such as China and Russia.

But Burgess said he also wanted to let the other country know that its spies had been rumbled, and that the unit's team leader had been confronted by Australia's own spies.

"We want the A-team to know its cover is blown. We want the A-team's bosses to know its cover is blown," he said.

"If the team leader failed to report our conversation to his spymasters, he will now have to explain why he didn't, along with how ASIO knows so much about his team's operations and identities."