I traveled 2000 km on the North island roads, 1000 km on my own and 1000 more by buses 🚙 In my opinion a car or a van is definitely the best way to explore the country👍🏻And do not worry about the left-side driving, it’s very safe. There are lot of signs and arrows right on the roads that help right-side drivers to adjust.
Car renting prices vary. I would recommend Jucy car and campervan. After exploring lot of options, they appeared to have the best price-quality value, the service is also very good! (and this is not an ad 😋) If you do not have required by New Zealand driving license translation, Jucy would do it for you for extra fee right in the office. First time left-drivers have to watch special video, it takes 15 minutes, and you are ready to go!
Second best option is to take special tour bus 🚌 that goes from one attraction to another, like hop-on hop-off across the country. There are passes for different amount of days (from 2 to 28+ days), the stay at one place does not have time limitations. Many young people and solo travelers select this option. You may find an example here, there are lot of similar companies suggesting such travel option.
I also tried regular buses 🚎 , one of them was comfortable, another one from 80’s, with just few time options – not the best way to explore the country. But if you are not limited by time, go for it! Note, that there is no connection between some cities, but hitch-hiking is well developed 🙌