This is probably my favourite local mountain bike ride. Great scenery, good gravel road - even a tunnel to go through!
Starts at Huia. Park at the first gate, and walk up the lower dam to the second gate. Then there is a rather evil hill to begin with, but after that it's a very gradual easy climb. About 6.2km (each way)
Alternatively, you can walk in from Piha Road to the north. This track can be quite muddy though and technical. About 4.5km. If you can organise it, a through trip from Piha Rd right through to Huia is a classic.