This covers all of the major routes we drove (it doesn't include all the excursions) on our nearly-three week trip to New Zealand this year. Starting with picking up the rental car at the Marlborough regional airport in Blenheim, and ending with our final destination in Dunedin.
There were a few overlaps with our first trip to New Zealand, but with a couple of notable exceptions we kept them to a minimum. For the most part, driving on the South Island is fantastic. The roads are well built, in good repair, and traffic is pretty minimal in most places.
The second time around, driving on the left side of the road was much quicker to get used to, and felt completely natural partway through the trip. I still got the turn signal and windshield wiper stalks confused on occasion, though - they're opposite of left-hand drive cars! So if you're in New Zealand and see a car going through a roundabout with its windshield wipers going on a dry day, you can bet it's probably a tourist.
And, of course, the scenery can't be beat.