A mother who killed three of her children in 2015 by deliberately driving them into a lake in Australia has been jailed for 20 years.

The 37-year-old mother, Akon Guode, admitted that she was behind the murder of her three children — one-year-old Bol and four-year-old twins Hanger and Madit. She had also tried to kill another child, named Alual, but the six-year-old survived.

Guode was put under custody on 16 January till her next court appearance on 31 January.

Guode had pleaded guilty to murder, infanticide and attempted murder. A judge said her motive behind the crime is still unclear.

Justice Lex Lasry told Guode in the Supreme Court of Victoria on Tuesday (30 May) that, "Your betrayal of their trust was catastrophic. People want to understand why you did what you did, because particularly for parents of young children such action is foreign and unthinkable."

"In my opinion, your actions were the product of extreme desperation, rather than any form of vengeance."

The judge said Guode had been suffering post-traumatic stress, showing signs of depression and was feeling isolated from the Sudanese community.

Guode arrived in Australia from South Sudan in 2008.

She entered her petitions with help from an interpreter and added that she drove the car into Lake Gladman in Wyndham Vale on 8 April, 2015, with the intent to kill the children.

In earlier court hearings, witnesses confirmed what had happened on that day. One onlooker said in an emergency call played to the court, "They (the children) are floating; they are floating in the water."

"(They) can't swim."

The father of the three children, Joseph Manyang, stated that Guode was a "loving mother". She would not have harmed the children intentionally. She had spoken of feeling dizzy before the crash.

A key witness however told the police that Guode spoke about killing her children on the day the incident happened.

Guode was sentenced to 26 years and six months in jail, with a non-parole period of 20 years.