Don't worry you'll be fine. Here are the the things to be aware of while driving on the left;
1) Here is the US, when you make a left turn you have cross oncoming traffic, in NZ it is a right turn. I have a little mantra when I drive on the left, I keep saying in my head "a right turn is a bad turn, a right turn is a bad turn....."
2) traffic circles. Not only are you not used to them in the US, you enter and exit them from the opposite side in NZ. Mentally run it through your head as you approach one, ie "which way am I going to turn, when I get there".
3) Suprisingly, highways can get a little confusing. The right lane is the fast lane and the left lane is the entrance and exit lane. However NZ has no highways, except in Auckland, so that won't be a big deal.
4) The biggest problem you will have isn't a big deal, and will even be fun. Every time you walk to your car you are going to walk to the wrong side and won't realize it until you open the door!
Botom line NZ is a great place to drive on the left for the first time. It is uncrowded and the roads are good. Have fun!