The bodies of Helena Wright, 36, and her 23-month-old daughter were found in waters near Cape Northumberland on South Australia's southeast coast on Monday after police were alerted to an empty car with its engine left running.
On Wednesday afternoon, police called off a three-day search of land and sea for Ms Wright's three-month-old son, who also vanished on Monday.
"Based on expert advice and available information, the infant could not have survived the period of time that has elapsed, either in the water or on land," a SA Police spokesperson said.
Prevailing weather conditions forced emergency vessels out of the water for much of Tuesday and limited search efforts on the shoreline and clifftops, "significantly" reducing the likelihood of finding the baby, police said.
"The reality is, after three days, the chances of finding the baby alive are zero," Superintendent Trent Cox said on Wednesday.
"We've been unable to … launch water assets. We've been able to get the SES aerial drone up," he said, adding that experts had advised against deploying an underwater drone in the dangerous conditions.
Despite calling off the search, authorities will continue to monitor the shoreline over coming days in the hope they can retrieve the baby's body.
Authorities continue to investigate exactly how the tragedy unfolded, after revealing on Wednesday they were treating the incident as a murder-suicide.
Ms Wright was originally from Estonia and had been living in Compton, a small locality outside Mount Gambier about 26km north of Port MacDonnell.
Police have confirmed she attended Mt Gambier Police Station the day before the tragic incident, where she spoke to police.
A call from a friend led police to conduct a welfare check at her home about 9.30am on Monday, but the home was empty.
Less than two hours later, at 11.15am, police were alerted to an abandoned car left with its engine left running near Cape Northumberland.
The bodies of Ms Wright and her toddler daughter were recovered from the ocean on Monday.
The pair was found below the cliffs and retrieved with help from local fishermen.
A coronial investigation is now underway, with a report to be prepared for the coroner.
Police have spoken to the woman's husband, the father of the children, and say the deaths are being treated as a suspected murder-suicide.
Earlier, officers said there was no information to suggest anyone else was involved in the incident.
SA Premier Peter Malinauskas said the tragedy had shocked the community to the core.
''If it involves children, that breaks your heart … particularly when we know there was a two-month-old baby missing. That's hard to get your head around," he said on Tuesday.
Major Crime detectives and specialist forensic officers remain at the scene as investigations continue.
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