Welcome to Puketutu Island.
Te Motu a Hiaroa (Puketutu Island) is a volcanic island in the Manukau Harbour, New Zealand, and is part of the Auckland volcanic field. European settlers called it Weekes' Island, but this was eventually abandoned in favour of the historical Māori name. The island is joined to the mainland via a causeway known as Te Ara Tāhuna. (Wikipedia)
You are looking for artificial stone located behind the post near small tree.
Good luck.