Mangawhai to the Bay of Islands
Take the coast road out of Mangawhai Heads and head up through Langs Beach to Waipu (25 km approx). Although it begins with a winding hill, this avoids the steeper hills on the main highway and is much prettier. The route curves along with coastal views past Langs Beach and Waipu Cove (store and camping, a great family beach) The road then heads into Waipu proper along a flattish straight.
Waipu is an old Scottish settlement and has most of the facilities you might require. Take the side loop heading north rather than heading for the main junction and this will take you eventually and unavoidably out onto Highway 1. There is a good shoulder however and after 4-6 km you will come to a DOC campground (no store, boil-your-water) at Uretiti. This is a lovely beachfront government camping ground among the pine trees and worth a night if you want a short day. If you want to press on, head up to Ruakaka (a further 5 km, store, campground). Another 30 km will take you into Whangarei, a large town, almost a city, with all facilities. Whangarei is the place to make decisions. Below is a fellow cyclist, Nic from South Africa... a bit of contrast to get an alternate view of the same journey.
This was mostly made up by highway and is not recommendable. Busy, lots of trucks and I even had a close call. I just couldn't figure out another way to do it practically. Whangarei itself I did not find very attractive but I guess this could be relative to where you are in the town. I went to a camping/backpackers that I just could not get myself to stay at as it felt too much like a motorpark and ended up staying at a Guest house called Chelsea House that was really great, an awesome 100 year old place where one feels right at home.
You can take a fairly arduous but beautiful series of side roads from here out to the coastal townships of Ngunguru, Matapouri and Whananaki and side roads to Helena Bay. This will take at least a couple of days, possibly three. It is probably best to grit your teeth and take Highway 1 and the 20 or so kilometres to Whakapara and then turn right at the junction that will eventually take you to the Bay of Islands (Russell). Let me quote Nic again for this part of the journey.
I stayed overnight on this route in an amazing cottage called Lucky Hill. The coastal scenery here is surely some of the most spectacular in New Zealand, with little islands and intricate shapes in the coastline all along the way. The hills makes for interesting times though, again, I found it more difficult to do this route than most of the routes I did in the South Island. It's up and down, up and down. Good thing it's so beautiful. From Russell I cycled up to Keri Keri Airport and flew back to Auckland as my time was up.