Potentially sensitive government emails may have been revealed when an international hacker group broke into Foreign Affairs Minister Murray McCully's email account.

Mr McCully's private Xtra account was hacked by the group, Anonymous, while he was overseas in April last year, Fairfax reports.

However, he said he did not believe there were any Cabinet papers or foreign cables among the emails in that account.

"Ninety-five per cent of the stuff that comes through on that account when I am travelling is media stories ... then there's a bit of office administration stuff," Mr McCully said.

"We discovered it because people started sending out emails from my account."

He said the outgoing emails mainly made fun at his expense but would not reveal further details.

Mr McCully had informed Prime Minister John Key of the security breach, prompting an investigation by the Government Communications Security Bureau.

Mr Key has warned other MPs to be more careful.

He said he was not sure what information the hackers had accessed, adding: "if it was really sensitive it would be out by now".

Anonymous has previously brought down New Zealand government websites in protest of copyright laws, along with hacking US government and company websites.