Blogger Cameron Slater has held back from releasing information about online donations to the Labour Party.
Mr Slater had said he would publish the details on his Whale Oil blogsite at 2pm on Monday, but is awaiting legal advice on how much of the donor list he can release.
Labour says a vulnerability in its system allowed files to be downloaded from a party website containing thousands of names and email addresses, as well as information about a smaller group of donors.
Labour claims Mr Slater has downloaded a number of back-up files from a website operated by the party.
In a letter to the blogger, Labour general secretary Chris Flatt says the files contain personal data and publication of some of that information would be unlawful under the Privacy Act.
Labour says no credit card details were held on the site.
Party president Moira Coatsworth says one of the earliest downloads of the database appears to be from a National Party head office internet address.
Mr Slater says donors to Labour have every right to be angry - but not with him, as it was the party that failed to protect the information.