The cache is located 40 paces down hill (following the path) from the Rata Street Reservoir. If you park at the top of Rata Street, you will reach the Reservoir after 5-10 mins walking. The cache is a hidden in plain sight 200ml Sistema Container, containing a log book and pencil.
For the more adventurous, the Cache can also be accessed, by following a path down from the ECNZ summit ridge track on the Eastern Hutt hills, but as trails are unmarked, access from Rata Street is definitely the easier option.