The largest (only) city on the South Island is Christchurch. By US standards it's quite small, but it's just large enough to have all the goodies you need - a great airport, wonderful places to eat, fantastic parks, good hotels...

The NASA Kuiper Airborne Observatory deployed to Christchurch each year to do observations of the southern sky. I was fortunate to have made multiple trips to Christchurch, one of my all time favorite places!

In all, I've spent at least six months there and enjoyed every minute of my stay.