Hello Rod,
Welcome and some good suggestions above.
Post offices: post shop hours vary from town to town. For further
information on hours and shipping see
http://www.nzpost.co.nz/ and/or
give them a ring. I have found they're staff to be very helpful.
Btw, depending upon where you choose to hike, you may find it easier
to buy as you go rather than send packages.
Sim cards: If your phone is unlocked, either Telecom (
telecom.co.nz)
or Vodafone (
vodafone.co.nz) have sim cards available with prepay
plans. Vodafone currently has a plan called Freebee that I have found
to be pretty economical... 2 Degrees is another mobile provider, but I
have no idea what their coverage is on or near the trail. If your
phone is locked, then there are phones available in NZ to buy, and by
law they have to be unlocked so you could take one back with you or
sell it.
I would suggest you flip on over to the TA Facebook page. That's been
the most active place for info and connecting with hikers, and there's
some good planning files there as well.
Good luck with your hike and I hope you enjoy your time in NZ!
Judith