CONVINCED her children were about to be brainwashed by their "cult leader" grandparents, a young mother decided her family's salvation lay in death.

Last April she placed her three children, aged 16, 10 and 2, in the family car and crashed head-on into a tree at 100km/h.

"I thought now was my opportunity - mainly for protection; not because I didn't love them, more that my mind had thought so much about all this evil," she told a psychiatrist in August.

"I had to protect them from being brainwashed ... the only way was to go to heaven."

Yesterday the Supreme Court found the woman, who cannot be identified, not guilty of attempted murder due to mental incompetence.

Acting Chief Justice Margaret Nyland made the ruling based on expert mental health reports which she later released to The Advertiser.

In one report, forensic psychiatrist Dr Craig Raeside said the mother, 33, had schizophrenia, with acute relapses caused by the use of illicit drugs.

He said the woman had delusional ideas about her adoptive parents wanting her dead because they were church-affiliated cult leaders. They attended court yesterday to support her.

Justice Nyland ruled the woman was mentally incompetent at the time of the offences and found her not guilty.

The woman is now liable to serve a limiting term - a period under mental health supervision equal to the jail term a healthy person would receive. Neither the children nor the woman was seriously injured in the incident, which occurred at Ingle Farm.

A subsequent urine test showed the woman had consumed methylamphetamine shortly before.

Justice Nyland said the limiting term needed to include conditions allowing the woman to interact with her children. Her contact with the children is now supervised by her parents.

"Obviously that is the aspect of the matter that causes me great concern," Justice Nyland said.

"If we are looking at the conditions of a licence in due course, what arrangements are made for the children?

"On the same token, the court would wish she had continued contact with them."

Prosecutors said they did not want the woman to be detained while receiving treatment.

The matter will return to court in March.

Originally published as Crash mum was fearful of 'cult'