A pearl oyster and spiny ornamentation resembles
Whether or not is is any of these species, or a related species, it is not known to be native to South Australia, and is likely an introduced species, from either the tropical Indo-Pacific, subtropical Australia , or the Mediterranean (NB the latter is included as possible, given the location next to an international shipping port).
Beneath the jetty, at the base at one of the deeper piles. Only one individual sighted (unlike Glenelg or Brighton, where they are plentiful)