Hi Steve,
Arrowtown - definitely a must and try to get accommodation there instead of Queenstown (expensive).
If you can plan for the right night (lunar phase) and can afford to stay there, the Hermitage resort at Mt Cook has decent newtonian scope and if the weather suits they pack guests in a minibus out to the airstrip for some observing. Consider taking binocs, camera and a tripod - anything - as this is an awesome dark site, one of the darkest I've ever seen. It can also be extremely cold at night as the cool air sinks off the mountains into the valley. I used my little compact to snap these:
http://www.dpreview.com/galleries/73...-cook-airstrip
http://www.dpreview.com/galleries/73...k-by-moonlight
The tramp up to the glacier is also well worth doing.
If you can time it to arrive at Lake Pukaki very early on a cloudless calm morning you might be lucky enough to snap this:
http://www.dpreview.com/galleries/73...lake-pukaki-nz