A mother has pleaded guilty to killing three of her seven children, and trying to kill a fourth, by driving into a lake at Wyndham Vale in Melbourne's south-west.

Akon Guode, 37, showed little emotion as she pleaded guilty to the infanticide of her one-year-old son Bol, the murder of four-year-old twins, Hangar and Madit, and the attempted murder of her five-year-old daughter Alual in 2015.

Akon Guode was driving when her car plunged into a lake at Wyndham Vale. ( Supplied )

The mother's plea in the Victorian Supreme Court came at a mention hearing before she was due to stand trial.

As the charges were read to her, through the help of an interpreter, the only words she said were "guilty" to each charge.

Passersby and emergency services frantically tried to help Guode and her children after her car plunged deep into Lake Gladman at Wyndham Vale.

Alual survived after being pulled from the water and sent to hospital.

A police statement publicly released in July last year alleged Guode spoke about killing herself and her children on the day of the tragedy.

A court hearing was previously told she drove by a lake at Wyndham Vale five times before "deliberately and consciously" driving into the water.

Guode's eldest daughter had testified that her mother had dizzy spells six months before she drove into the lake.

Mother escaped war in South Sudan

Guode came to Australia from South Sudan to escape war after her first husband died.

She 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 earlier told Guode became too frightened to interact with the Sudanese community.

Mr Manyang described Guode as a caring and loving mother.

During a court hearing Guode collapsed and wailed as a harrowing triple-0 call from the incident was played in court.

Following the deaths of the children there was an outpouring of grief from the local Sudanese community.

Passersby frantically tried to help Guode and her children after her car plunged into the lake. ( ABC News )

ABC/AAP