The History
The residential site above you was once owned by the Colonial Sugar Refining Company. In 1928 the old Farm building's that had been there prior were demolished. Despite this loss, the refinery was welcomed by locals due to the provision of much-needed jobs during the Depression. The facility officially began operations in May of 1930 and supplied all of Western Australia’s crystal and liquid sugar for the next six decades. The refinery’s red silhouette became a classic landmark for sailors and the Swan River, and with public access limited, biodiversity in the area was preserved.
The requirement of an extensive upgrade or replacement, and the changing value of residential land resulted in the Colonial Sugar Refining Company selling the plant to developers in 1995. Demolition was instant, transforming the land into the high-end real estate development that exists today.
The cache
The sistema is hidden down near the old pump house remains, around 10m South West. Access from land is not easy, but not impossible. It requires some clambering down from the walkway above, or rock hopping along the coast for about 150m if accessed from Point Roe Park. I've rated the difficulty level accordingly. Access via some sort of water craft would make it considerably easier.