Cossey's Reservoir is a walking track at Hunua in Franklin, with a Google rating of 4.9.
A good walking track is well marked, clear underfoot and honest about how long it takes and how hard it is. If you are choosing a walk, check the track length, the surface and any steep or muddy sections before you set out, and allow more time than you think you need. In the Hunua area, bush tracks can be slippery after rain, so sturdy footwear is worth it, and the weather can change quickly, so bring a waterproof layer. Regulars carry water, tell someone where they are going and start early, so they finish before the light fades. Many tracks in the Hunua Ranges are managed as regional parkland, so it is worth checking for current closures and track conditions, and following any signs about protecting kauri and other native plants by cleaning your shoes.
For walkers around Hunua and Franklin, Cossey's Reservoir is a very highly rated local track to explore.