Did both islands two years back, small borrowed car in the North, LWB Transit Campervan in the South. The roads are fine, and units are KPH. The only motorway I remember was travelling through Auckland.
The main roads are labelled SW (state highway). Mainly single carriageways that open up to dual at certain sections to allow for overtaking. One hugely annoying thing was that folk drive along at 50mph, then along comes an overtaking section and it's like a license to speed. I struggled to overtake in my aunts 1 litre Sirion, although my aunt received an $80 fine for a speeding ticket after I got home, luckily they didn't endorse her license. It's a lot like driving the A9 to Aviemore from Perth.
The South island is a lot more hilly and the roads more twisty, but following the same routine as the north. Overtaking in a 2.5 TD Transit is a lot easier than the "heat pump" I had in the north (my kiwi cousins term for the Sirion). I did get pulled for running a stop sign in the middle Greymouth on Boxing Day. No traffic, rolled through at walking pace and the cop told me "Stop means Stop!" Luckily, no fine, but that's another story.
You'll love it, the drive down the west coast of the South Island is tipped as one of the best drives in the world. Good mountain biking near Hokitika.