A mother who killed her preschooler because she believed she and her daughter were possessed has been deemed insane at the time of the killing and won't stand trial.

Evelyn Sen appeared at the High Court at Auckland in a hearing set to determine her level of mental health at the time of the death of her daughter, Maggie Watson.

Justice Mathew Downs described it as a "sad case" and said there was "no evidence the defendant was anything other than a good mother".

"The defendant killed her four year-old daughter but was insane when she did so."

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He reached the "allied conclusion" Sen should be indefinitely detained as a special patient at the Mason Clinic, he said.

Sen will not stand trial and is now legally deemed not guilty of murder.

She cried softly in the dock while her parents, in the public gallery, looked on.

Maggie's body was found at Sen's Onehunga home on August 7 last year.

Sen was not arrested and charged until six months after the four-year-old's death. She had pleaded not guilty and was set to stand trial in November.

A pre-trial application was made by Sen's lawyer Stephen Bonnar QC for the court to determine whether she was insane at the time of her daughter's death.

The court heard evidence from experts who had met with and interviewed Sen and their evidence was presented to Justice Mathew Downs.

'OH MY GOD, I CAN'T LIVE WITHOUT HER'

In his judgment, Justice Downs said Sen had given Maggie an extremely high dose of anti-depressant medication - 134 times the usual dosage.

She then gave herself an overdose and tried to cut her wrists.

When she called ambulances at 4am on August 7 she was said to have been "extremely distressed".

Told her daughter was dead, Sen was said to have replied: "Oh my God, I'm stupid. Oh my God, I can't live without her."

She told paramedics there were demons in her house and she was possessed by them.

The court heard Sen tried to get help for her mental illness as far back as 2012, when she told her doctor and family and friends that she felt a demonic presence in her house, that she thought she was being followed by police, and people were conspiring against her.

Among her beliefs were that police were tracking her, that her home was under surveillance by a satellite dish, that an intruder was frequently in her home, that there was a demon in her house or some other presence.

She believed her neighbours were conspiring against her and told others of rituals her church group performed, including her belief that a cross had been burned into her forehead.

She told people of a "strange sensation" entering her body through her shoulders and leaving through mouth and believed she had been injected with needles that filled her with anxiety and fear.

In 2015 she travelled to Malaysia to visit her family and took Maggie with her. Her family were so concerned with her mental state that they took her to a local healer to "exorcise" her.

"That person thought [the] defendant deeply troubled, however nobody seems to have thought Maggie was in danger," Justice Downs said.

Months later, Maggie was dead.

POSSESSED BY A DEVIL

Psychiatrist Mhairi Duff told the court that Sen suffered from delusions, and had come to believe that her daughter was possessed and being "tortured for the purposes of evil" after hearing Maggie screaming during the night.

She could not bear her daughter's suffering and did not believe Maggie was simply having nightmares, the court heard.

Sen had previously come to the conclusion that she should kill her daughter and herself but on one particular day, after a morning at the beach, Sen told a psychiatrist that she realised with "clarity" that it was time to carry out the plan, the court heard.

"[Sen believed] if she killed her daughter that her daughter would die an innocent and be assured a place in heaven, and accepted her own fate of going to hell as she recognised killing herself would be morally wrong, but not that killing her daughter would be wrong," Duff told the court.

Sen took another overdose after gaining consciousness and realising Maggie had died but she hadn't, the court heard.

"She continues to ask herself why it was that she lived and her daughter died," Duff said.

Asked to explain the decision later, Sen said she must have been possessed by a demon, the court heard.

Prior to that day there was no indication that Maggie had been neglected or abused, Duff said.

Sen believed her alleged actions were "morally right" in the circumstances she found herself in, but now understood it was wrong and was "highly distressed" about it.

"I must have been demonically possessed. The devil got inside my head. It was wrong," Sen was said to have told Duff.

Justice Downs refused applications to film and photograph Sen in court, after opposition from both the Crown and Bonnar, who said it would be "bordering on inhumane" to photograph her when she was at a time of "intense emotional anguish".