Maggie Watson's mother found not guilty of her murder

Maggie Watson (supplied)
Maggie Watson (supplied)

An Onehunga woman who killed her four-year-old daughter last year has been found not guilty on the grounds of insanity.

Evelyn Sen believed her daughter was under threat from demons and that killing her was the only way to save her.

Maggie Watson was given a deadly dosage of mertasapine, an anti-depressant only suitable for adults.

Her mother gave her 134 times the usual adult dosage of the drug believing she was being tortured, and her soul was at risk of possession by demons.

"I am satisfied that the defendant suffered a disease of the mind at the time she killed Maggie," the judge ruled on Thursday.

Sen suffers from Schitzophrenia.

The court was told she planned to take her own life as well in the hope it would send her daughter to heaven, even if it meant she went to hell.

Sen was deemed incapable of knowing right from wrong. She became visibly upset in the dock when Justice Downs read out his verdict.

Sen's parents were also there, having travelled from Malaysia for the hearing, and became equally upset when they heard the details of their daughter's illness.

Defence lawyer Steve Bonnar said it was a "tragic case".

"These are never happy situations but obviously the court was satisfied that the appropriate verdict was not guilty by reason of insanity."

Sen was also troubled by beliefs she was being tracked by police, and that her house was under surveillance.

She'll now be detained as a special patient at a forensic psychiatric facility.