A mother who killed three of her children by driving them into a Melbourne lake told police she loved her children but she was not well.

Akon Guode, 37, has pleaded guilty to the infanticide of her one-year-old Bol, and the murders of her four-year-old twins, Hanger and her brother Madit.

The mother of seven also pleaded guilty to the attempted murder of her six-year-old daughter Alual, who survived.

Her Supreme Court plea hearing has been told that at the time she deliberately drove her 4WD into a Wyndhamvale lake on April 8, 2015, she was depressed and in debt.

She was also isolated from her Sudanese community because of an affair she had with the father of the four children inside the car.

In her first interview with police, Guode told officers she loved her children.

"I just want to tell you that I love my children," she said.

"This terrible issue happened to me because I'm not well."

The court heard Guode had a traumatic birth with her baby son Bol but maternal health services deemed her to be of "low risk" on a post-natal assessment.

The plea hearing before Justice Lex Lasry resumes on Thursday.

© AAP 2017