This is a no so quick cache find here. It is on the Mount Sommers walkway and is not exactly a easy track up to GZ.
I tend to have a walk up to the summit at least twice each year and it is such a wonderful 'little' walk, it usually takes me around 2.5 hours to get to the top however the day I set the caches out it was nearer 3 due to putting them out for you. There is a very nice summit plateau however to get there you need to negotiate a narrow track and a very steep climb to get to the plateau.
The track also circumnavigates Mount sommers and has a couple of DOC huts around the 'back' so you could spend the night out in the wop wop's. below is a link to the DOC website for more detailed information.
http://www.doc.govt.nz/parks-and-recreation/places-to-go/canterbury/places/mount-somers-area/things-to-do/tracks/mount-somers-track/
this cache is a 200 ml sistemia container with a bit of cammo tape on the lid. It is right next to the footpath, just look for a rock on the top. The GPS was telling me it was a 4 meter accuracy with a height of 640 meters above sea level.