You can avoid the worst of the Dargaville traffic by heading out along the quieter Hokianga Rd, which soon becomes Waihue Rd, forking left at Parore West Rd to join Highway 12.
Soon after Kaihu (Service Station, pub) you have the choice to continue along the highway, but I recommend instead turning right into the quieter Trounson Park Road. No sooner have you turned off when you come to the "Kauri Coast" Top 10 Holiday Park along the banks of the Kaihu River. Here the promise of a guided night-walk through the forest may entice to stop early for the day. As the road signs attest, this is Kiwi country and if you are lucky a night walk may reward you with a sighting of these shy, nocturnal birds. At the very least you're likely to hear their surprisingly raucous cry when night falls.
Another 5 or so km brings you to Trounson Kauri Park, a forest reserve with a DOC campground and bush walk through Kauri forest. The camp, while a little close to the road, is pretty flash for a DOC offering, having a kitchen with cooking facilities, a fridge and even showers which are purportedly hot. This is a good place to stop and perhaps enjoy your own DIY night-walk.
Personally I wanted a bit more ride out of the day so after pausing to fortify myself with a spot of lunch I pressed on along the now gravel road through the tiny hamlet of Donnolly's Crossing and up to rejoin the smooth tarmac of highway 12. The reward was a wonderful 5 km downhill run through the luxuriant Waipoua Forest.
At around the 50 km mark you reach the turn-off to the Waipoua Visitors Centre. The centre is a kilometre down the road, with a café and an excellent campground set along the banks of the Waipoua River. A great place to stop for the night.