The Labour Party is investigating what it calls a "malicious breach" of its website after right-wing blogger Cameron Slater began to release inside party information he had obtained.

Labour's president, Moira Coatsworth, said yesterday that the party was concerned personal information of donors and members could be used in a "politically motivated" way after an online contact database was "exploited" through a weakness in security.

She said one of the first downloads of the information appeared to be from a National Party head office internet address, and it was subsequently accessed by a person with strong links to National and Act - understood to be Whale Oil blogger Mr Slater.

On his blog yesterday Mr Slater said he would progressively release information over the coming weeks. He claimed to have emails, financial details, planning information and membership data. He said he had done nothing illegal to obtain the data.

Ms Coatsworth said the database did not hold credit card details, but was a list of people who had donated to the party and its campaigns, or joined up as members through the website.

Party secretary Chris Flatt yesterday emailed all those on the database, apologising and assuring them the system was now secure and an independent security review had begun.

The Herald on Sunday yesterday published the first of the information released by Mr Slater - minutes from a meeting of the Labour North branch. They included some campaign planning, including the intention of ensuring Labour mobilised voters for the Te Tai Tokerau byelection.

The minutes also spoke of using parliamentary resources to secure "the best outcome for LP [the Labour Party]" but included a reminder that parliamentary resources could not be used for campaigning.

Mr Slater said he would ask Speaker Lockwood Smith and Parliamentary Services for a full investigation of the use of Parliamentary Services staff and resources for campaigning and party business.

By law, parliamentary funding can not be used to solicit votes, donations or members and its use in election periods is more widely restricted. In practice, parliamentary staff often take part in party business, although they must do so in their own time.