Police have confirmed a child they are looking for in a major search operation off South Australia’s coast is three months old.

The bodies of the boy’s mother and sister 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 girl who died was under three years old, while the mother was a 36-year-old from Compton, near Mount Gambier.

Police believe the younger child is missing in the water.

Tall waves and low visibility in the water could hamper search efforts today but Police Commissioner Grant Stevens said emergency services would continue to scour the region.

SES volunteers walk along a rocky beach with calm waters

The search was a joint operation between police, SES, CFS and volunteers. (ABC News)

“Despite the weather today which is not good, searching is continuing for the infant,” he said.

“There are no indications, at this point, of any specific location to search. The CFS, SES and police and volunteers are doing an exceptional job to try and locate this small child.

“As you can imagine, searching for a small child and recovering a woman and a child from the water will take its toll on people who are involved in that search exercise. 

“They are genuinely committed to finding this young child as quickly as possible.”

Grant Stevens in police uniform in front of SA Police banner

Police Commissioner Grant Stevens says emergency services are “genuinely committed” to finding the missing infant. (ABC News)

Yesterday, police said they held “grave concerns” for the safety of the missing baby as local fishers described the “treacherous” coastline may make the search difficult.

Police won’t speculate on what happened

The search began after an officer responded to reports of a car with its engine running in the car park at Cape Northumberland about 11:15am on Monday.

That officer then spotted the bodies of the woman and the toddler in the water, some distance offshore.

The commissioner said the investigation remained active and would not “speculate or try to predict the outcome”.

“We do a comprehensive investigation which will look at any interactions with police, other support services as well — at this time it would be inappropriate to elaborate,” he said.

“We have spoken to the husband of the woman and the father of the children.”

SA Premier Peter Malinauskas said he was briefed by the Police Commissioner on Monday night.

“It’s very difficult at this point in time to understand exactly what transpired here,” Mr Malinauskas said.

“There are a number of theories that the police are actively investigating but wherever we see the loss of anybody it’s a tragedy. 

“Where it involves children, that breaks your heart. We’ve got to be careful not to jump to conclusions.”

Police said there was no information at this time to suggest anyone else was involved in the incident and reports would be prepared for the coroner. 

An Isuzu car.

Police released this image of an Isuzu similar to the one found at the scene. (Supplied: SA Police)

They have urged anyone who may have seen the car that was found at the scene — a white Isuzu MU-X, with South Australian registration S592 BJR — to contact Crime Stoppers.