A depressed mother strangled her five-year-old son at home before fatally stabbing herself near Bukit Timah Nature Reserve, a coroner's inquiry has found.

The boy was autistic and had attention deficit hyperactivity disorder, and Japanese national Nami Ogata, 41, was stressed about him. She also told doctors she could not cope with work and caring for her two children.

Madam Nami left a suicide note addressed to her husband, apologising for her actions and telling him to take care of their younger son. She then drove to Lorong Sesuai to take her own life, bringing her son's body with her.

State Coroner Kamala Ponnampalam ruled Madam Nami's death to be a "deliberate act of suicide" and her son's death to be an "unlawful killing at the hands of his mother" in her findings which were made available yesterday.

The findings state that Madam Nami had sought medical attention for her low mood and anxiety, and had told a psychiatrist about her suicidal thoughts and feelings of depression at least three days before the fatal incident on Nov 14 last year.

She told the doctor she had been anxious for the past few months and had been sleeping badly. She also had loss of appetite, weight and hair, and had palpitations for about a week.

She told the psychiatrist she was stressed about her son Sotaro, without revealing the exact reasons, the coroner found.

Madam Nami was immediately referred to the emergency medicine department at Singapore General Hospital, where she denied being actively suicidal but said she had been feeling depressed for a year.

She was discharged with drugs to treat her insomnia and was to be reviewed by her private psychiatrist the next day.

Madam Nami subsequently told her private psychiatrist that she "would not harm herself on account of her children", even though she had "one transient, short episode of thinking of ending her life a week prior", stated the coroner's report.

She was diagnosed with major depressive disorder and given antidepressants and a tranquilliser.

Madam Nami wrote in her suicide note that the medication did not work and told her husband "she was going to take Sotaro" with her.

Her husband had left Singapore for a business trip to China on Oct 30 and had not sensed anything unusual when he last spoke to her on Nov 10, according to the coroner.

Madam Nami also wrote to her brother, pleading with him to raise her younger son together with his children.

According to her maid, on the night of Nov 13, Madam Nami read storybooks to her children until they fell asleep. She later texted the maid to tell her that she had taken Sotaro to the hospital as he was "having a fever".

However, police investigations showed that Madam Nami had likely strangled Sotaro in the living room using a long elastic band and raffia string.

Around 5.40am, she drove out, with her son's body covered in a white blanket, to Lorong Sesuai.

The two were pronounced dead at the scene by paramedics.