The posted co-ordinates are not where the cache is. It is at another sweet spot located some distance away. There is no need to do anything mathematical although if you did you will get within a few metres of it. Just pick up your GPS device and go for a walk. The cache is within 5m of a walking track. If you come across a tree trunk lying parallel to the track you have gone too far down. Give time for you device to settle down.
S45 50.959 E170 29.611
S45 50.966 E170 29.826
S45 51.078 E170 29.792
You are looking for a screw top container of approx 75ml containing only a logbook. No room for swaps. Bring your own pen.