Lake Alexandrina is a coastal freshwater lake located between the Fleurieu and Kangaroo Island and Murray and Mallee regions of South Australia, about 100 kilometres (62 mi) south-east of Adelaide. The lake adjoins the smaller Lake Albert (together known as the Lower Lakes) and a coastal lagoon called The Coorong to its southeast, before draining into the Great Australian Bight via a short, narrow opening known as Murray Mouth.
You will be looking for a non magnetic mint tin placed in the uprights along the Mannum pipline no need to climb under or over the pipe as the cache can be easily found on approach.
{FTF} joint effot to Bear771511 and Plesiosaur22 for getting it @1826