Breaking: Search for missing baby boy continues off SA coast

Emergency service continue a search for a baby boy near Port MacDonnell. (ABC News: Aiden McNamara)

Police have confirmed a child they are looking for in a major search operation off South Australia's coast is "about" two months old.

The bodies of the boy's mother and sibling were found floating in the ocean yesterday, after authorities were called to Cape Northumberland, about 400 kilometres south-east of Adelaide.

Authorities have also confirmed the child who died was under three years old.

Police believe the two-month-old is missing in the water.