Police said the ship was passing Cappa Jaffa at the time.
PolAir and an Australian Maritime Authority Challenger aircraft from Melbourne are currently conducting the search.
The cruise ship is also assisting the search, which is being conducted in five metre seas, while local SES crews are on standby.
"The cruise ship is also assisting with the search in the 5-metre seas and local SES (State Emergency Service) are on stand-by," SA Police said.
A Carnival Australia spokesperson said the woman's family member was "being cared for by our onboard team while every effort is being made to find this guest".
"This is an incredibly distressing situation for all involved including for our remaining guests and crew," the spokesperson said.
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