I finished the tramp on a day that some would call miserable.  I don't call it miserable, I call it weather that is common to Stewart Island.  The island is beautiful because it does get so much rain (4-5 meters a year) and if you are in a place with that much rain.  You will get rained on.  It can be guaranteed at least one day out of the 8-10 days it takes to tramp will be wet at least part of the day.  Most likely, it will be more than one day.