It takes an hour or two to get acclimated and a whole lot longer to get comfortable. Touring the the countryside is no problem - just stay focused and make sure the center line is on the same side of the car as the steering wheel. It's the cities with the big roundabouts that freak me out.
I used to have to fly into Sydney, pick up a rental car, turn left out of the Alamo parking lot, get over into the right lane, turn right at an intersection about 3 KM up the street, hang another right a couple of blocks up from that, then turn left into the office parking lot. Simple enough but those two right turns required a physical force of will not end up in the oncoming lanes but after that I was usually OK.
Having driven a stick shift for the past 35 years, would it be safe to say that I should rent an automatic if I ever venture to NZ?.....LOL