A woman who killed her three youngest children by deliberately driving her car into a lake, in what a Supreme Court judge described as a "horrendous crime", has been jailed for 26 years and six months.

But Justice Lex Lasry ruled on Tuesday that Akon Guode's life was in turmoil on April 8, 2015, when she drove into Lake Gladman in Wyndham Vale with four of her children in the car, and that there was a connection between her disturbed mental state and the offending.

Guode drove four laps of the lake before she did a U-turn, steered through the only entry point to the lake and accelerated into deeper water.

She then got out but did nothing to help residents and emergency services workers who frantically tried to save the children.

Four-year-old twins Hanger and Madit and 16-month-old Bol all drowned, but their older sister, now seven, was rescued.

"Your actions amounted to a horrendous crime on innocent children and a gross breach of trust and a betrayal by you of your obligation to protect your children," Justice Lasry told Guode.

Guode, 37, pleaded guilty to two counts of murder, one of infanticide and one of attempted murder.

She cried through much of her sentencing – at one point Justice Lasry stood down for five minutes – and must serve 20 years before she is eligible for parole.

The charge of infanticide applies to women who kill their children who are younger than two when affected by mental-health problems related to childbirth. The infanticide charge relates to Bol's death.

Guode's lawyers had urged Justice Lasry to view all the offending through the infanticide prism given she was a mother of seven in financial trouble whose mental health had declined since she lost a lot of blood during Bol's birth. 

Guode was also socially isolated after being ostracised from Melbourne's Sudanese community because of her affair with Joseph Manyang, the father of her four youngest children.

A psychologist earlier this year told the court Guode was likely to have had a major depressive disorder and post traumatic stress disorder from her life in Sudan, where her first husband was murdered.

She and her three eldest daughters came to Australia as refugees in 2006.

On Tuesday, Justice Lasry  said much of the case remained a "tragic mystery", as Guode had not explained exactly why she killed her children.

She initially claimed she had suffered a dizzy spell, the court heard, but neurological tests showed no problems.

Justice Lasry said Guode had lived an "extraordinarily difficult life" and that cases such as hers tested the community's compassion, as people wanted to understand how parents could kill their children.

"In my opinion your actions were the product of extreme desperation rather than any form of vengeance of a kind that has arisen in other cases of people killing their children," he said.

However, the betrayal she showed her children was "catastrophic", the judge said, and left Mr Manyang with an unimaginable loss. Guode's eldest daughter now cares for her younger sisters.

Justice Lasry said he did not consider Guode's failure to help her children as an aggravating factor, because she was most likely in shock.

Despite her crimes, she was not considered a future risk to the community, and instead someone he hoped would be treated with compassion.

Guode's committal hearing last year heard she believed Mr Manyang's wife used witchcraft on her, and that Guode did not want to lose her children.

When committed to trial she pleaded not guilty to three murder charges, but in January pleaded guilty when one count was withdrawn and replaced by infanticide.

Justice Lasry imposed a 12-month jail term for infanticide, making Guode the first woman jailed on the charge in Victoria. Other women who have previously pleaded guilty to the charge were spared jail.

Guode has served 660 days in protective custody and faces deportation to Sudan once released.