Hi Tony. First off - great choice!
Alan makes a point worth noting about the rain. Currently experiencing around 3" per day down there! Though it would be different in Feb, is good idea to be aware and make allowances.
The Xpan could be a good option if you are confident you can switch to pano format images intuitively - not knowing experience there. I think pano format would suit the start of the treck - thru the river deltas - but perhaps less so later in the higher ground. But your views may well be different! Weight wise, I think the Xpan could be very good option, though it is a relatively heavy 35mm camera.
In my experience it's definately all about the #1 the weight #2 the space you have #3 efficiency of gear for images required.
If you're going to carry LF gear on a 3 day tramp, it would be very helpful to be pretty fit so you can cover ground quickly to make a vantage point at optimum light which may only become apparant when you are out there. The extra gear required is significant too don't forget. Change tent, film holders, boxes for exposed film (I carry separate boxes for N,N-,N+ and N++ b&w development) as well as new. Heavier tri-pod presumably. How many lenses would you carry - I never carry more than 3 on a longer hike.
Flexibility in the timetable also helps. If you can allow the option to stay an extra night or two to make the most of a location, this can mean the difference between success or not. DOC's estimated walk times are quite easy but aren't for photographers! As an example, a couple of weeks ago I stopped on a tramp in the in the central plateau and spent two hours photographing some vocanic hot springs, completely unplanned and arrived at the first hut after dark. The stop contributed a lot to the overall trip.
BTW I think on the Milford track there's the option to have someone carry your gear.
Cheers, John.