As always in rural NZ the surface is a constant surprise. This route is only recently tar sealed and (on this occasion) still lots of gravel patches but they should clear quickly.
Outside of the holiday season I'd expect little traffic except locals so you'd need to be on the lookout. There was also a stock drover with a small mob of cattle.
This can be done in a day with stops, OR go slow and stay in Bay of Islands or the Hokianga. Must visit Arai Te Uru in Hokianga just out from Omapere, also Waipoua Forest there (Tane Mahuta kauri tree). More traffic near Paihia but very quiet elsewhere. The route mapped is longer, but can be shortened by staying on the main highway (boring but fast). The basic route can be done in 3hours...everything is very close! Fuel in most places but not from Dargaville to Omapere.
This route is twisty in most places, and remote. Often no phone signal, but beautiful views and decent road surfaces with little traffic. Tourists can peel off for Kohukohu to catch the ferry across the Hokianga Harbour (beautiful), and continue on another amazing area of roads through Waipoua Forest etc. If you like gravel, head to Whangape/Pangaru/Mitimiti where its very local and well-isolated beaches. Fuel up in Kaitaia.
RIDE ALONG THE ENTIRE BEACH! It doesnt show a road on the map along the beach, but its a public road and you can ride from Ahipara to Te Paki Stream at the very north past Matapia Island. You can do this as a loop or there and back along the beach, which is pretty but also very straight and flat for 85kms each way. ANY BIKE CAN RIDE THE SAND...but be careful of your lines if on a dedicated road bike. I have ridden both bikes, and my Ulysses has road tyres. Make sure you ride the tide line and fuel up in Ahipara or Kaitaia as there is nothing else other than the odd fuel truck at Pukenui or Waipapa Landing. Dont get stuck on the beach, its an expensive mission to get out and most of this area is out of phone coverage. The SH1 route is windy and fun, with not many police (ive seen one in all my years and we have a place at Ahipara). Go slower between Ahipara and Waipapakauri, as its used by locals. Respect the beach area as its sacred (Te Ara Wairua, path of the spirits).
This route can be broken up or done completely in a few hours. Dont bother with SH1 (main route between Wellsford and Warkworth), unless short of time. I couldnt get rid of the map line! A side trip to Tawharanui Beach is worthwhile, as is Goat Island Marine Reserve in Leigh. Plenty of corners and relatively quiet roads, occasional police presence but not much if any. As its a local ride for me (living in Whnagaparaoa), I cant remember ever seeing a cop more than once or twice.