Jan and Bill's place is a lovely apartment that will make you feel at home for whatever duration of stay you have in mind. (I just stayed there for 6 weeks while I was working at the University of Auckland.)
The location is perfect if you want to be just outside the main city of Auckland but within easy reach of the center. It takes not much more than 30 minutes to walk into the heart of the city, and there are also regular buses to take you there. Herne Bay itself is delightful. An easy walk along Jervois Road takes you to the busier area of Ponsonby Road. Herne Bay is more laid back and replete with great restaurants and coffee shops.
Jan and Bill are wonderful hosts and excellent communicators. I couldn't recommend them more highly.