North Island Cycle Touring

New Zealand is a hilly country so cycle touring of necessity involves a good deal of hill climbing. The map link below gives you an  idea of the topography of New Zealand.

http://earthobservatory.nasa.gov/IOTD/view.php?id=5227

The choice of routes is determined by the topography and the condition and the suitability of the roading network for cycling. There are not always secondary roads to ride on because of the topography but in some areas there are secondary roads and these should be used by cyclists rather than sticking to main highways.
One relevant statistic is that 76% of the population on New Zealand live in the North Island. This suggests that cycle touring in the North Island is always going to be more difficult than in the South Island. If I had a month to tour by bike in New Zealand I would always choose the South Island because of its population density and its unique scenic highlights. This recommendation comes from a cyclist who lives in Auckland.   

In the North Island if you are traveling from Auckland to Wellington the east coast is a good option although the west coast is also interesting. From  Auckland one option is to go down the east coast and head to Tauranga and Gisborne and then continue down through the Wairarapa to Wellington. This is a good route with its share of hills with the ride to the East Cape and Gisborne being a highlight.

When heading south out of Auckland the recommended route if you are going south to the east coast is to take the ferry from downtown Auckland to Pine Harbour and to begin riding from there to avoid the traffic and the lack of roads with adequate shoulders.

Route South from Pine Harbour to the East Coast

The ride from Auckland up to Northland is well worth doing but the main road north out of Auckland is generally not suitable for cycle touring so an alternative route is required for cyclists. There is an alternative route to use that goes through Helensville and up the west coast.

http://newzealandcycletouring.blogspot.co.nz/2015/03/cycle-touring-out-of-auckland-north.html 

The same applies when going south down through Hamilton on the west coast. There are alternative routes going through Tuakau and riding on the western side of the Waikato river rather than on the Waikato expressway.

Roads in the North Island not suitable for cycle touring

• Auckland to Hamilton - Waikato expressway - cyclists are permitted on this road and it has adequate shoulders but it is very busy and it is not a pleasant place to be.

• State Highway 5 - Napier to Taupo - high traffic density with limited shoulders
• State Highway 1 - Auckland to Whangarei
• State Highway 1 - Levin to Wellington

Note: This list is not an exhaustive one and are only the roads that I have experienced