Akon Guode has been sentenced to 26 years and six months' jail for killing three of her children, and attempting to murder another, when she drove them into a lake in Melbourne's outer south-west two years ago.

The 37-year-old had pleaded guilty to the murders of four-year-old twins Hangar and Madit, and the infanticide of 17-month-old Bol, after driving her four-wheel-drive into a lake at Wyndham Vale in April 2015.

She also pleaded guilty to the attempted murder of five-year-old Alual, who survived after she was pulled from the water.

Prosecutors did not seek a life sentence over the killings.

Justice Lex Lasry said Guode, who survived civil war in South Sudan, suffered post-traumatic stress and symptoms of depression, but the reason she drove her children into the lake remained unclear.

"The real reason for your actions on this day are in many respects a tragic mystery and have not been explained by you," he said.

"As in other similar cases, this case tests the sympathy and compassion of the community.

"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."

Guode will serve a minimum 20 years' jail before being eligible for parole.

Rescuers attempted to save Akon Guode's children after the crash. ( Supplied: Supreme Court of Victoria )

Sentence 'inadequate' for crime

The court had been told no mother had ever been imprisoned for infanticide in Victoria, which carries a maximum penalty of five years in jail.

Justice Lasry said he did not believe Guode was a threat of re-offending.

Akon Guode will "very likely" be deported after she has served her sentence. ( AAP: David Crosling )

"The sentence I'm about to impose is in some respects inadequate to reflect the gravity of what you've done, yet at the same time excessive given your mental state and your background of hardship and desperation," he said.

"These are obviously grave offences. These children trusted you as their mother as they were entitled to; your betrayal of that trust had catastrophic consequences."

Guode had to make three deliberate turns of her steering wheel to navigate through the only open access point towards the lake, and one witness had described seeing Guode's four-wheel-drive going "full bolt into the water".

The court was previously told that at first, witnesses who called triple-0 did not think the car was deep enough in the water to be of any danger to the occupants.

Several people ran down to try to rescue those inside and observed Guode standing nearby the driver's door "not saying or doing anything".

One witness used a steel-capped boot to smash the rear window and freed the submerged baby Bol from his child seat.

But the baby, Hangar and Madit could not be revived.

A number of emergency services personnel were in the gallery of the sentencing.

Akon Guode suffered severe psychological trauma from the Sudanese civil war, the court was previously told. ( Supplied )

In sentencing Justice Lasry paid tribute to the first responders, including police, ambulance officers and firefighters, as well as civilians, who were confronted with the scene.

"Such a task is incomprehensible ... we too often take the essential services these people provide for granted, they are to be thoroughly commended for the professionalism for their response to this incident," he said.

The defence had previously told the court Guode had suffered severe psychological trauma from the civil war, which had been left untreated for two decades.

Justice Lasry said Guode had lived "an extraordinarily difficult life, a life that most of us can hardly imagine".

He said she spent 18 days fleeing South Sudan with three children, surviving attacks by hiding in the bushes, after she witnessed the murder of her husband.

The sentencing of Guode had to be briefly adjourned as she became inconsolable as the judge relayed the details of her life.

He said Guode would "highly likely" be deported upon release from prison, which "compounded the tragedy" of the case.

She had been having a long affair with the father of the children involved in the incident, Joseph Manyang. He described Guode as a caring and loving mother.

Mr Manyang separated from his wife after Hangar and Madit were born and the affair became public, but lived separately from Guode and her children.

The court was earlier told Guode became too frightened to interact with the Sudanese community.