The Government's secretive surveillance agency believes Foreign Minister Murray McCully's private emails were broken into by the international hackers' collective Anonymous, despite claims by a self-described Russian hacker ''Yuri Petrov'' that he was to blame.

In February Fairfax Media revealed the April 2011 breach of Mr McCully's private Telecom Xtra account, which had been used to forward official emails.

Documents released by Prime Minister John Key under the Official Information Act today show that a report from the Government Communications and Security Bureau (GCSB) identified ''cyber activities attributed to the group 'Anonymous','' had resulted in Mr McCully's private webmail account being compromised.

Prime Minister John Key said he was not able to say much about the issue.



However, the GCSB had told him the hacking of McCully's email was carried out by Anonymous.



He had never heard of "Yuri Petrov".



"The advice I have had is that the group that was responsible was Anonymous - who makes up that group - that's for another day."

Last May Mr McCully's senior private secretary sought advice from GCSB, saying: ''You'll be aware that recently the minister's personal email address was hacked by the 'Anonymous' group.''

''We are looking for a balance between ease of use and security for his account.''

Prime Minister John Key's chief of staff Wayne Eagleson said other information related to the hacking was being withheld because publication would risk prejudice to the security or defence of New Zealand. It was also subject to an obligation of confidence on GCSB's part.

Advice circulated to Beehive staff informing them of the hacking and giving advice on improved security was also withheld.

Mr McCully has said the breach took place while he was overseas and the account was closed down. ''We discovered it because people started sending out emails from my account.''

They had been about ''a whole range of things'' but mostly making fun at Mr McCully's expense.

Sources said government agencies still believed Anonymous was behind the hacking, but their focus was on the effect of the hacking and preventing a repetition.

The documents show GCSB defined Anonymous as ''a self-organised group of WikiLeak supporting net-citizens that banded together to form a large group''.

Anonymous expert, Canadian academic Gabriella Coleman, said it was ''a cluster of ideas and ideals adopted by various, sometimes unconnected hackers, technologists, activists, geeks, and human rights activists, and grounded in the concept of anonymity''.

Its activities had ranged from pranks to technological support for revolutionaries in Tunisia, Egypt and Libya.

To back his claim ''Yuri ', who said he was leader of a group called ''The Comrades'', released a handful of the hacked emails that appeared to be genuine.

They included references to the location of Defence Force planes, criticism of the Foreign Affairs Ministry, and details about a New Zealander arrested on drugs charges in South America.

He had insisted he was emailing from ''south of Moscow'', but an expert who analysed his messages said he was certain they were sent from a New Zealand computer, leading to speculation a disaffected foreign affairs staffer was to blame.

''Yuri'' has recently established a Russia-addressed email account, and has pointed to a website that appears to show a series of security breaches around the world.

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