The attack on Murray McCully's personal e-mail account took place in April, when Anonymous was protesting the country's copyright law, The Dominion Post reported Tuesday. McCully had been having e-mails from his official Cabinet account forwarded to the compromised account, heightening concerns, the report said.
"They were forwarded on from the official system, but it was not about the Ministerial Services e-mail system," Prime Minister John Key said.
McCully said the breach occurred while he was out of the country and the account was closed down.
"We discovered it because people started sending out e-mails from my account," he said. "Ninety-five per cent of the stuff that comes through on that account when I am traveling is media stories ... then there's a bit of office administration stuff."
McCully said he didn't know what specific information was accessed by Anonymous but "if it was really sensitive it would be out by now."
Key's spokesman said there are no protocols in place to prevent cabinet members from having work e-mails forwarded to personal accounts.