A mother accused of murdering her five-year-old son and trying to drown another of her sons was delusional and thought killing her children would save them, a court has heard.

Key points: Woman pleaded not guilty on mental health grounds

Woman pleaded not guilty on mental health grounds Court heard she had a history of drug and alcohol use

Court heard she had a history of drug and alcohol use Court also heard her surviving son told police his mother "put my head in" the Murray River

The 28-year-old woman's murder trial began in the Supreme Court at Wagga Wagga on Monday.

Defence council Eric Wilson told the court the woman — who cannot be named for legal reasons — thought she and her children were going to be killed by a former partner, and that the only way to save them was to kill them.

The woman is accused of drowning the five-year-old boy and trying to drown her eight-year-old son in the Murray River at Moama on March 2, 2017.

She does not dispute that she killed her son, but pleaded not guilty on the grounds of mental health.

The court heard the woman had a history of drug and alcohol use as well as a history of sexual, physical and verbal abuse throughout life.

She was released from custody in February 2017, one month before the incident, and had stopped taking medication for mental illness after leaving jail.

'My mother told me to put my head in'

Crown prosecutor Max Pincott told the court that after the incident police interviewed the older boy, who told them: "My mother told me to put my head in and she dunked me."

Mr Pincott told the court that after the incident, the mother went to a nearby resort where she told the owner she had drowned her babies, and had tried unsuccessfully to drown herself.

The court heard she told the resort owner: "I had to do it, I had to do it, I had to drown my babies."

Mr Pincott told the court the resort owner took the woman to Echuca Police Station where she told officers she had "killed her kids".

The court heard the mother later told police: "I didn't want to do it, I am sick, I will lose my soul."

Police removed the five-year-old's body from the Murray River near Moama after the incident. ( ABC News )

Mr Pincott told the court that during the mother's interview with police, she admitted she tried to drown the then-eight-year-old and held her younger son underwater until he stopped struggling.

The court heard the eight-year-old sustained extensive bleeding and soft tissue wounds from a dog attack in the water.

The 28-year-old woman's defence is that she was mentally ill, and that she could not stop herself killing her son, even though she knew it was wrong.

Her lawyer said the mother was delusional, she had fear that her former partner was going to kill her and her children and she thought she was keeping them safe.

The court heard there was nothing to suggest the woman's former partner posed any risk to her, or her children.

The judge-alone trial is set down for a week.