Toonice. Let's see if we can figure this out. As you have activated the 3rd party stuff correctly (via the Scenery Library) I suggest you deactivate the problem areas first. Then go to the area to make sure the problem is not with default textures.
I just checked out GZR Biggin Hill & it works fine. To clarify the Priority issue with GZR scenery, GZR Base 2001 includes files common to many other bases so must be activated first or arranged below (higher Priority number) all other GZR airfields on the Scenery Library menu. The best place for it would be immediately above the default scenery areas. (Although I confirmed Biggin Hill works with GZR Base 2001 I would normally use the later GZR Base 2002. Not sure where I found it.)
I can't check out the Auckland NZ stuff without knowing where to find it. A lot of 3rd party scenery uses textures from the Airport, VOD, Nova & other standard texture sets posted at SimV & most FS sites. (Texture sets, not the design programs themselves.) These textures are intended to be installed to the CFS2 main Texture folder. I also suggest you install Gerrish Gray's tree library.
http://www.simviation.com/fsdesign_scenery6.htm It should be mentioned in the Readme with a specific scenery if these are required but it's a good idea to have them.
PS. GZR Base 2002 here.
http://www.simviation.com/fs2002scenery1.htmUse either this or the original GZR Base 2001, not both.