Hi Tim,
Hope you had a great trip up to the Bay of Islands a couple years ago. Just an update for anybody else reading this thread however. We have a brand new, world class Mountain Biking Park opened late 2016 in Waitangi Forest (only about 4km from Paihia). It is called the Waitangi Mountain Bike Park and has professionally constructed trails with proper signage and a range of single track trails from Beginner (Grade 2) to Advanced (Grade 5+). Currently there are around 15 to 20km of tracks, but building is ongoing and the plan in the next couple of years is to increase this to 70 or 80 kilometers of single track. It is a great place to ride already, and will only get better and better. Park owners ask for a donation of $10 per day to ride, which goes straight back to the trail building fund, however it is usually smarter to buy a 1 year pass for $25, which gets you heaps of sweet deals and discounts around town so you will no doubt make it all back in the same day over dinner or something. Also, bit of a plug, but if you are after some decent full-suspension mountain bikes to rent while you are up here so you can have a spin out on the trails, please email Paihia Mountain Bike Rentals at paihiamtbrentals@outlook.com and we will be able to tee something up for you :). Hope to see you riding up here soon anyway. It is less than 3 hours from Auckland too, so not too far.
Also, the local Mountain Bike Club up here is the Kerikeri MTB Club, it is pretty active and currently meets most Tuesday nights at Inlet Road, Kerikeri, or longer weekend rides are sometimes planned too. There are some local club built MTB trails over there too which they use. Cheers, Jonny