We just went for a walk down town. It's 10:30pm but most of the stores are still open, and there are people EVERYWHERE, kids and adults, dressed up for halloween. All of the pubs/clubs are having halloween themed events - it's like the whole town is having a late-night festival. It's fantastic!
So I have an addendum to my recommendation for the perfect NZ trip: Come to Queenstown in the last week of October. Totally awesome!