Pranav Vivekanandan is missing after the bodies of a woman and child, believed to be his mother and brother, were discovered in Yerrabi Pond on Saturday morning.

Police have taken away a white Subaru from the banks of Yerrabi Pond in Gungahlin and will continue to search for a young boy there on Sunday.

Police said: "While an extensive land and water search for Pranav has been under way, police and family are issuing this plea in the event that he is alive and someone knows his whereabouts.

"Pranav is described as being of Indian subcontinental appearance, about 110cm (3'7") tall, with short dark hair, and dark brown eyes. He may have been wearing an orange T-shirt and light-coloured shorts, and a jacket.

"Serious concerns are held for Pranav's welfare."

The car - with a baby seat in the back - was said by locals to be at the edge of the water since about 4am.

Rod Wheatley said on Facebook the car was blocking access for a truck at 4.12am. "I opened the gate, noticed it was not locked with a child seat in back. I actually opened the door to see if keys (no keys) were in it, to move for access."

The police were told and it's understood the vehicle was being examined as part of the investigation of the death of a mother and her son, and the disappearance of the boy's brother.

ACT Policing said it was an ongoing investigation. About 30 police officers were involved, including divers and a drone operator. Emergency services workers were also at the scene.

"This is obviously a tragic situation," Detective Superintendent Hall O'Meagher said.

The cause and manner of the deaths was not known at the time. Homicide detectives were involved but it was not clear whether they were seeking an outside perpetrator of the deaths.

The efforts of the police were concentrated on the search for the second brother and on talking to the family of the dead mother.

"We have a land search under way and also a search in the pond itself," the police superintendent said.

"We are making enquiries with the families of the deceased. Our thoughts go out to them and also to the responding police who have had to deal with this tragic circumstance.

"We are asking for any member of the public who has information in relation to the circumstances of this matter to come forward and call Crime Stoppers on 1800 333 000".

A police statement said: "A crime scene has been established, and ACT Policing homicide detectives have commenced an investigation."

The water was said to be murky and not easy to search. It is a water settlement pond used for recreational activities, said Darron Marks of the Friends of Yerrabi Pond group, which has been campaigning for better lighting around the water. He said the pond was not deep, usually about a metre in depth.

The police were also using a drone to get a better view of the wider area.

Police said there was no danger to the wider community.

Counselling was being made available for officers and SES personnel involved.

Police were not saying what the possible background to the tragedy was. It is not known if the deaths were an accident or from some other cause, but the police did not appear to be mounting a serious crime operation with a hunt for some outside perpetrator.

There was no appeal for information about any other specific person but the police did want anyone with any information which may be useful to contact Crime Stoppers on 1800 333 000, or on the Crime Stoppers ACT website.

A report will be prepared for the coroner.