The police "seizing" material from the news media isn't that big a deal. [Update: except for the bit that is ...]
Just a very quick post on what I think the Police are up to in serving search warrants on news organisations for material relating to "Teapotgate" (there you go, Steven Price).
Reading this TVNZ story, it appears to me that the desire is not to recover copies of (or transcripts of) the actual recording itself. Rather, they are seeking evidence relating to the circumstances in which that recording took place. So, they'll be seeking from Radio New Zealand the master recording of Mary Wilson's interview with the cameraman Bradley Ambrose (I guess an on-line version is not good enough from an evidential perspective), as well as any notes she may have compiled during any pre-interview conversations. And TVNZ has been asked for its raw footage of the Key/banks meeting, no doubt so the police can see exactly whereabouts the media were when the recording took place.
[Update: In response to a comment below, I'm revising this. Trying to get any pre-interview notes from Mary Wilson (or other Checkpoint interviewer) is problematic, given the need for the media to be able to protect their sources. This isn't a cast-iron reason to never disclose such material, but it does raise issues that aren't involved with just getting recordings/general footage of the event.]
Therefore, I'm guessing the Police still are at the stage of assessing what evidence (if any) there is that an offence has been committed under s.216B of the Crimes Act. To decide this, they need to determine whether the conversation was a "private communication", as well as whether any interception (recording) of the conversation was "intentional". It's only if they decide there is strong enough evidence of an offence under s.216B to warrant charges that they'll start getting interested in who (if anyone) has disclosed the content of the recording, in possible contravention of s.216C.
As I've posted already, and argued in the comments thread here, I really don't think there has been any offence committed here at all. But I guess that when the PM alleges that there has, the police have to take that allegation seriously and investigate it properly.
[Update: According to this, Bradley Ambrose is seeking a declaratory judgment that the Key/Banks conversation was not "private". I assume this means that it was not a "private communication" in terms of the Crimes Act, and thus not unlawful to record it (whether intentionally or not). It's not clear, however, he'll get such a declaration ... especially now that the Police are actively investigating the matter, and deciding it may involve issues of fact that a Court may decide a jury ought to determine.)