The cache is a black taped approx 200ml round pill container containing a log book only. Bring your own pen.
Inside the lid of the container are two letter/number combinations that you will need to solve the final location of the Waiatarua Mystery. The co-ordinates are also written on the log bag, and there's a couple of laminated bits of paper in the cache with them on too.
This is a nice little section of woodlands that a lot of people familiar with the park often haven't been into. There are quite a few well worn trails in it, so although I've never seen anyone in there myself, a few people must walk through. You can walk through from one side to the other, crossing the stream/river on the way. When it's been raining a lot, it's more river than stream :)
FTF: funkymunkyzone & harisree
2TF: Mikkyp
3TF: tuolei
Update 25 November 2017: with all the planting they've done in the area on the car park side has a lot more cover so it's less obvious than it was before where to enter. I've added waypoints to help you find your way in. The hint contains info about how to find it from the car park side.