A riddle:
Whene'er rain falls upon this hide
the water flows to either side:
The Onkaparinga to its west,
and Bremer east it finds its rest.
Between each basin's highest peak
lies the fortune that you seek.
Use this GeoCheck link to check your solution.
Cache is a small Sistema-like container including a pen.
It is recommended you park at S +0.519', E -0.345' from the cache site. You can also approach from the north if you hate fun and spectacular views.
Update 2019-10-18: Added hints for common incorrect solutions on GeoCheck and added to the hint here