Auckland to Mangawhai
First off, you will need to catch a train that heads out west and alight at Waitākere Station. This is generally not a problem and they run fairly frequently
Let us assume you have arrived at Waitākere Station from Auckland and it is 11.30 am. Your next point is Helensville (camping ground, hostel, supermarket etc). This is approximately 45-50 km and can be reached by using the back roads through to Kumeū via Taupaki and thence the back road from Kumeū to Helensville. There is a main road, Highway 16, but this is a busy thoroughfare and while there is a broad border in parts, you are often left on a narrow fringe of tar-seal with traffic passing close by at high speed. The back roads are very pleasant and allow a little sightseeing of small rural holdings and a background of pine forested hills.
From the Waitākere Station, follow Waitākere Road through Taupaki till it meets up with the main road at Kumeū (about 4.5 km). Cross the main highway and take Oraha Road. This climbs up (about 2 km) to a T junction with Old North Road. Take a left onto old North Road and continue for 5 or so kilometres until you reach another T junction. Take the left onto Wishart Road and this will lead you into Helensville. The roads are not always clearly marked Avoid any deceptive side roads masquerading as T junctions including Pinetone Road, Ararimu Valley Road, Inland Road, Waikoukou Valley Road, Peak Road, Taylor Road and Kiwitahi Road. The route gives good views over rolling farmland and the Kaipara Harbour. If this sounds complicated, it isn't really and you can always ask locals if you are unsure.
Helensville is a small rural town with all facilities and has a history of river traffic because it is bordered by the vast Kaipara Harbour on its western side. Helensville could have been the capital of New Zealand but the shifting sandbars of the Kaipara proved to be too treacherous for shipping. A few kilometres out of town is the mineral spa of Parakai Springs. This is worth a look if you like hot water bathing/soaking.
Alternatively, you can skip Helensville and catch an Intercity bus from Auckland to Wellsford (or get dropped at the Mangawhai turnoff) and start your journey there.
This is a longish day, not because of the distance, but because there are a series of steepish rises and falls on the road to Wellsford and these can be tiring. Take Highway 16 out of Helensville heading out toward the small township of Kaukapakapa (12 km). The road passes through farmland and affords great views over the Kaipara Harbour. Keep following the road for another 46k's to Wellsford. Wellsford is a large town on Main Highway 1. There is a accommodation here at the pub, but no campground. Continue following the main highway for about 6 k's or so passing through the township of Te Hana and then turn right onto the road to Mangawhai (approx 24 km).
Mangawhai has several campgrounds, stores and hostels. My favourite is the Riverside Camping ground just before the town. You can camp on the estuary and watch the moon rise over the water. To your right are the great dunes of the bird sanctuary. To your left the mangrove swamps and the inland river. There is a further campground at Mangawhai Heads, another 7 km or so that is right on the mouth of the estuary looking out toward Sail Rock.
Mangawhai will likely be your introduction to Northland's famous beaches. A coastline of fine, yellow sand and clear waves that are tinged with phosphorescence at nighttime. You might like to take a break for a day here before pressing onward. If you can, hire a kayak and head over the estuary to the bird sanctuary, part of a great sand bar bordering the coast. You can spend a day just moseying around here or walk across to the beach and swim. It's a long, beautiful beach and mostly empty. Mangawhai is called magical by the locals and there is some evidence for this. Be aware that Mangawhai consists of two villages some six or seven kilometres apart; Mangawhai and Mangawhai Heads.
These observations are courtesy of Nic, a fellow cyclist from South Africa...
Once you get off the heavy trafficked highway 1 the Mangawhai road is much less busy and quite beautiful as you pass over the rolling hills made up mostly by farms. Preferably one would try to avoid the highway altogether. The scenery is serene and calming but not really comparable to what is further up north. There are some swim friendly beaches and Mangawhai Heads offers some surfing spots, but it seems a little spoilt being too populated (by New Zealand standards).
The funny thing about the North Island is that there seem to be fewer mountains, but more climbs on the bike, as you are always going up or down over the hills. This makes for some hard climbs with a heavy bike. The road is also narrow but the sparse traffic makes it better.