I drove a mini truck (3.5T) recently from AKL to DUD and I found them extremely accurate - even for trips ~3hours (i.e., within a couple of minutes). Being in the truck, I was limited to 90km (which I stuck to) and surprisingly had very few vehicles overtake me on the trip. BUT, I was on State Highway 1 (the main road in NZ) the whole way, so they are the best driving conditions. It woudln't be representative of many of the South Island roads.