The information contained in the PM is often in other places - straight away, someone else knows the information, and chances are, one of the two people it involves has shared the information in some way. In saying that, I'm not trying to say that you can't access PM's at all. Under the guidelines for the forums, it says that where something illegal is being done via the forums/PM's, action can be taken - that's a disclaimer to allow access by 3rd parties in the case of suspected illegal activity.
If the information is "lost" there are legal channels you can go through to get it - our government has to be transparent. I'm not sure that the anti terrorism laws apply when MAF is the one implementing them, and it's really really hard to prosecute under them. Also, bio terrorism would only apply when there's terrorism involved - that is, an intent to create fear in the wider community. So, smuggling a snake, for example, wouldn't be bio-terrorism, but smuggling 200 with an intent to release them to cause fear in NZ would be.