East of Auckland near Hikuai, the Grapevine Track area has several 4WD tracks open to offroad motorcycling.
A good place to start is the Gravel pit located near Hikuwai. This is about 15km from the turnoff to Whangamata on Kopu-Hikuai Road.
Note: It is best to ride this area when dry, and even then, do not ride alone.
Overall, Grapevine Track is the 21st most popular dirt bike trail of all 41 dirt biking rides in New Zealand.