The best ones we did were Parihaka MTB trails in Whangarei. There’s not a great deal of trails here, but they are fun. We also rode at Waitangi Forest, but there was no singletrack. The Kennett brothers’ book recommends those two plus the following:-
-Tutu Trail, Ahipara 27km return, intermediate grade. It says you need to check the tides.
-Kaitaia Walkway, near Kaitaia, 12km return, intermediate grade.
-Twin Coast Cycle Trail, Horeke to Opua, 80km one way, beginner. Sounds like a rail trail.
-Glenbervie Forest, 10km northeast of Whangarei. Bike park with intermediate and expert trails (grades 3-5 trails in the book are graded 1 to 6, 6 being the hardest). Says theres 10km plus of trails. The book is a few years old, so could be more now.
– Whanui Loop, 15km east of Whangarei. 17km, grade 3, 70% gravel road, 30% singletrack.
All except Glenbervie get 2/4 for fun factor. Glenbervie is 3/4.
I’d recommend heading further south if you can to Rotorua and Taupo. Just had our first ride at Craters of the Moon this morning, it was great.