The three deaths in Yerrabi Pond were a murder by the mother and then the suicide of the mother herself, police believe.
Detectives believe it was not an accident but a tragic series of crimes.
The police said: "The case, at this early stage, is being treated as a murder-suicide with only the three deceased involved. However, because the investigation is complex and has not been completed, the circumstances surrounding the deaths have not been the subject of a detailed report to the Coroner.
"As the investigation is not complete it is not appropriate to release any further information at this time. It is the responsibility of the coroner to make findings as to the manner and cause of death. Those findings will be produced publicly when the coronial investigation (and if necessary, a hearing) has been completed."
The investigation began on Saturday morning when one of the bodies was spotted in the water.
A search revealed the second body. Investigators discovered the young deceased boy had a brother who was also thought to have been there.
The police named the missing boy as eight-year-old Pranav Vivekanandan. Pranav's body was found in the pond the next day.
About 30 police officers and 50 ACT State Emergency Service personnel were involved in the investigation, including divers and a drone operator.
"This is obviously a tragic situation," Detective Superintendent Hall O'Meagher said at the scene.
A white Subaru was spotted near the water by a member of the public before dawn. It was blocking a truck entrance.
The member of the public said: "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."
Yerrabi Pond is not a recreational pond. It is about a metre deep but muddy. It is not a swimming venue.
Counselling has been made available for officers and SES personnel involved.
The police said: "If anyone is having difficulty with this news, they are encouraged to seek support through specialist services such as Lifeline on 13 11 14 or Kids Helpline on 1800 55 1800."
Support is available for those who may be distressed. Phone Lifeline 13 11 14; Mensline 1300 789 978; Kids Helpline 1800 551 800; beyondblue 1300 224 636; 1800-RESPECT 1800 737 732.
