I'm actually surprised to find there are vehicles running on petrol there at all. New Zealand is tiny by American standards. You can't go 1000 miles in one direction without falling into the ocean from the furthest points, and in some places it's only 14 miles from coast-to-coast.
They have an extra tax per km for diesel which means it's roughly the same price for cars as petrol. Trucks are all diesel though.
Outside the city, there are very few decent motorways. The geography is quite complex. There are very few flat bits. And just outside the city there are plenty of gravel roads. They keep promising to tarmac them, but only a few km per year. Every year we have slumps and slips as the earth makes its way downhill. Most of the money gets spent on repairing them, and the big roading projects tend to favour the highways near the city.
It might be just over 1000 miles from end to end, but there's hills most of the way and a three hour ferry ride.