A mother who killed three of her children after driving her four-wheel-drive into a lake in Melbourne's outer south-west has had her sentence reduced by eight-and-a-half years on appeal.

Key points: Three of Akon Guode's four children drowned when she drove into a lake in Wyndham Vale in April 2015

Three of Akon Guode's four children drowned when she drove into a lake in Wyndham Vale in April 2015 She pleaded guilty to the murder of twins Hangar and Madit, 4, and Bol, 16 months, and the attempted murder of Alual, 5

She pleaded guilty to the murder of twins Hangar and Madit, 4, and Bol, 16 months, and the attempted murder of Alual, 5 The Court of Appeal has downgraded her sentence from 26 years and six months to 18 years' jail

Akon Guode was sentenced to 26 years and six months' jail last year after pleading guilty to the murders of four-year-old twins Hangar and Madit, and the infanticide of 16-month-old Bol.

The three children, along with their five-year-old sister Alual, were in the car when Guode drove into a suburban lake at Wyndham Vale in April 2015.

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

But the Court of Appeal today downgraded the sentence to 18 years' jail.

The 39-year-old will now serve a minimum 14 years' jail before she is eligible for parole.

The Victorian Supreme Court previously heard Guode had suffered severe psychological trauma from the civil war in South Sudan, after watching her husband being murdered.

One-year-old Bol (left) and twins Hangar and Madit Manyang where killed. ( Supplied )

In sentencing her at the time, Justice Lex Lasry said Guode had had an "extraordinarily difficult life" but the reason she drove her children into the lake remained unclear.

He said the case had tested "the sympathy and compassion of the community".

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

The Court of Appeal said it accepted the balance of Guode's mind was "disturbed" after the birth of her youngest child, but the sentence was "manifestly excessive".

"Major depression impaired the applicant's capacity to exercise appropriate judgement, and her capacity to think clearly and make calm and rational choices," it said.

"Had adequate weight been given to the applicant's mental condition and other factors in mitigation, we consider that significantly more lenient sentences would have been imposed."

Rescuers attempt to save the children of Akon Guode in April 2015. ( Supplied: Supreme Court of Victoria )

The Supreme Court was told Guode had to make three deliberate turns of her steering wheel to navigate through the only open access point towards the lake.

One witness described seeing the four-wheel-drive going "full bolt into the water".

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

Akon Guode suffered psychological trauma after experiencing violence in South Sudan. ( Supplied )

One witness smashed the rear window of the car and freed the submerged baby Bol from his child seat.

But Bol, along with Hangar and Madit, could not be revived.

Guode had been having a long affair with the father of the children involved in the incident, Joseph Manyang.

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 told Guode's life was "in turmoil" before the crash.

She suffered from a major depressive disorder after giving birth to her youngest child and had been threatened with violence by members of her social circle.

During sentencing last year, Guode became inconsolable as the judge relayed the details of her life.