The Official Information Act is one of our great constitutional achievements. It lays down a very clear basic principle for all government information: that information shall be made available unless there is good reason for withholding it. Various things can count as good reason, including prejudice to foreign relations or the fact that the information is legally privileged (and that privilege has not been waived), but those reasons must be assessed individually for each request in the circumstances when it is made. No information is excluded from this assessment; it is a harm-based regime, not a class-based one.
Now MFAT, one of the last bastions of feudalism in our government, have a new way of preventing scrutiny of the actions they take in our name: agreeing to secrecy with the other countries they negotiate with. One example of this is the Trans-Pacific Partnership, where MFAT have just agreed to keep all documents except the final text secret for four years.
This agreement is not expressly contrary to New Zealand law - but it is certainly contrary to its spirit. And it undermines that harm-based regime and perverts it into a class-based one. It doesn't matter if a TPP document will cause harm to our international relations or not; the fact that it is a TPP document is now reason enough to keep it secret until 2015.
(Alternatively, you can view this as MFAT creating reasons to keep things secret, in that having agreed to secrecy, release would be seen as breaking our word and harm our international relations. But that's only the case because MFAT agreed to it in the first place. If they didn't, then there would be no harm (or rather, it would depend on the document and the circumstances at the time)).
Its just another example of MFAT's undemocratic mindset and the way it locks us out of our own foreign policy. By doing this, they undermine the legitimacy of any deals they make. But MFAT still seems to operate in a monarchical world where the only opinions that matter in international relations are those of governments, not the people they purport to represent.