For a public official 'acting in the public interest' means acting lawfully and/or in accordance with government policy under the direction of the Minister. In the absence of legal or policy frameworks, the public interest means acting for the good of the community.
the common good or welfare of all. In practice it would be difficult to find complete agreement on what is in the "public interest." Once one gets beyond generalities and platitudes (it is not in the public interest to allow drunk drivers on the highway) one comes up against differences in the values people hold; sometimes by appealing to the public interest politicians try to universalize what are merely personal beliefs and values (or the interests of a section of the community) that may not in fact find common assent. See also national interest.
The interest in obtaining official information that sheds light on an Agency's performance of its statutory duties because the information falls within the statutory purpose of the FOIA to inform citizens about what their government is doing.That statutory purpose, however, is not fostered by disclosure of information about private citizens accumulated in various governmental files that reveals nothing about an Agency's or official own conduct.