Howdy Christian,
origin_pacific_ai_traffic_2006.zip is the latest Origin flightplans available from AvSim.
This pack contains two PAI Jetstreams (31 & 41) in OP livery and a traffic .bgl. Both AC and traffic file come as .exe files, so I simply pointed them at my .tmp folder, unzipped, and manually moved the two ac directly to my FS Aircraft folder and the traffic file to Scenery/World/Scenery. Works for me.
As for Mt. Cook, found them in the latest "Air New Zealand Group Winter 2005/2006". This is by "Perficad", available from Avsim or
www.carledwards.eu.com.
AC needed for the Mt. Cook schedule is the "ATR-72-500 Air New Zealand Link" and I've found that the best available is the ARNZ version by Jon Murchison (all Jon's ac work a treat if one uses Nick Needham's LightFix Version 2, this replaces the hideous bulbous FS2000 navigation lights).
Do remember though, it is
imperative that the title of the AC (found at the top of the aircraft.cfg file) matches that of the AC given in the traffic aircraft .txt file. If it isn't, then you get an AI no-show. (Personally, I change the traffic aircraft.txt file to be the same as the title in the aircraft.cfg file. YMMV, of course.)