The Inspector-General of Intelligence and Security will investigate claims the Security Intelligence Service (SIS) declassified information and gave it to a blogger for political purposes.
Cheryl Gwyn said an investigation was warranted, as there there was sufficient public interest in the matter.
Nicky Hager's book Dirty Politics alleges the SIS fast-tracked the release of information to the blogger Cameron Slater so he could attack former Labour leader Phil Goff.
It released documents about a 2011 meeting between SIS director Warren Tucker and Mr Goff to Mr Slater a day after he requested them.