Buy a Backcountry Hut Pass. You can't use it on a "Great Walk" but there are almost a thousand huts to choose from: Mueller Hut, Brewster Hut (a must), Granity Pass Hut and Mt. Owen (another must, trust me on this one)... the South Coast Track, Blue Lake... Paparoa NP: Fox River to the ballroom. Loop to Punakaiki on the Pororari River Track... Sawcut Gorge north of Kaikoura... Wye Creek Basin, The Remarkables Conservation Area... the Routeburn is very scenic but be sure to do a side trip to the Valley of the Trolls.
Some links to keep you occupied:
Rent a car from Scotties: http://www.scotties.co.nz
$20/day with an airport pickup. Can't go wrong.
Drive on the left side.
Turn signal on right, wipers on left of steering wheel.
Sign up for a shoppers discount card at Countdown.
Drink Hummingbird coffee. Bring a plunger (Bodum or french press).
If you drink wine, ditch the bottle and bring a platypus to carry the wine in.
Get a pack liner.
Carry an extra freeze-dried meal for just in case.
You almost always wear your boots through streams.
Bring hiking poles. Bring gear from home.
Wear a hat, sunscreen (extremely high UV level).
Bring earplugs for the huts.
Grab your bunks as soon as you get in.
Bring a few cuphooks and 6' of cord for hanging wet clothes, etc.
In older huts, hang your food up in a bag (mice).
Get a backcountry hut pass.
Buy camp-stove fuel at the hardware store (Mitre 10).
Don't leave anything visible in your vehicle when parked at a trailhead.
Have a great trip!