The Government is refusing to deny it called on the country's top spy agency to help lever a senior Cabinet minister into a plum overseas job.

Explosive revelations from United States-based website The Intercept allege the Government Communications Security Bureau used its specialist spyware to snoop on Trade Minister Tim Groser's rivals for a job as the World Trade Organisation director-general.

Groser's bid was seen as a long shot and he failed to make the final cut but the Government poured its resources behind him in an effort to secure international backing for his candidacy.

It now appears it also sought to give him an advantage over his rivals by using the GCSB to spy on international diplomats, including his rivals, and intercept email traffic referring to his chances.

READ MORE: Claims GCSB spied on WTO candidates 'disturbing' (Opinion)

Documents obtained by The Intercept show that, leading up to the May 2013 appointment, the GCSB set up a programme to snoop on emails about the other candidates from Brazil, Costa Rica, Ghana, Indonesia, Jordan, Kenya, Mexico and South Korea.

In what it labelled the "WTO project", the GCSB looked for emails referring to Groser, the WTO, and the surnames of other candidates. It targeted the Indonesian candidate in particular.

The claims are the latest in a series based on documents obtained by former National Security Agency (NSA) contractor Edward Snowden and were published yesterday.

The Government has refused to confirm or deny them, but the veracity of other files obtained by Snowden has not been seriously challenged internationally.

A spokesman for the WTO in Geneva said the office of the director general would not be commenting on the allegations.

Opposition parties are demanding an immediate inquiry by the Inspector General of Intelligence and Security and say it is unacceptable that the GCSB was used to further a Cabinet minister's personal ambition.

They have also questioned the use of the GCSB to further other diplomatic pursuits - including New Zealand's seat at the United Nations Security Council.

The revelations appear deliberately timed to overshadow Prime Minister John Key's visit to South Korea, where Key has faced embarrassing questions over the likelihood the GCSB snooped on a South Korean rival, Taeho Bark, for the WTO job.

Key said the issue had not come up during his discussions with senior government officials and claimed the South Korean Government "won't give a monkeys" about what the GCSB got up to.

Labour leader Andrew Little said the use of the GCSB was "highly dubious" when it was meant to combat security threats.

"This is outrageous. The GCSB is a foreign intelligence agency, not a recruitment agency. For all we know they also wrote his CV.

"These actions are a massive misuse of an agency which should be focused on our security threats, not the future employment prospects of a minister," Little said.

"As the minister in charge at the time, John Key must explain how he or anyone at the GCSB thought this was an appropriate use of the agency's resources."