Lauren Dickason psychotic at time of alleged murders - expert

The mental state of Lauren Dickason when she killed her three little girls is at the heart of her murder trial, which is being heard at the High Court in Christchurch.

The 42-year-old denies the charges against her, using the defences of infanticide and insanity.

The Crown said she murdered her daughters in a calculated way, because she was angry and resentful.

But the defence's forensic psychiatrist, who's an international expert on parents who kill children, has today given evidence that supports the defence's argument the mother was insane at the time.

"At the time of her offending Dr Lauren Dickason was labouring under a disease of the mind to such an extent that it rendered her incapable of knowing her acts were morally wrong," Dr Susan Hatters-Friedman said.

Her diagnosis of the defendant was that "[she] presented with symptoms consistent with a major depressive disorder with psychotic features".

Hatters-Friedman said Dickason's actions were "absolutely related to her mental illness".

"Through her mentally disturbed thinking, she conceptualised the joint filicide/suicide as saving the children, the right thing to do for her family.

"She perceived that she and the children would go to heaven together, indicating she did not believe this was morally wrong."

The expert believes eight deleted internet searches on how to kill children did not show planning.

"Even though she searched this up, this isn't how she went about doing it. So the method that she used that day, in an impulsive way, had nothing to do with these searches."

A Crown expert, who's set to give evidence on Monday, is expected to say that comments Dickason made in her police interview about why she killed one of her twins first showed she was angry.

The video of her being questioned in the Timaru Police Station was played to the jury on Tuesday.

"The first twin (Karla) was being really, really, really horrible to me lately. She's been biting me and hitting me and scratching me and throwing tantrums 24 hours a day, and I just don't know how to manage that. That is why I did her first," Dickason said.

Hatters-Friedman said she interpreted that comment differently.

"How I read this is, the one twin, Karla, is more physically aggressive, and the other twin Maya is more docile so it makes sense that the person who is going to be fighting back you're going to kill first... rather than killing the more docile one first while you're getting hit at by the other one."

As Crown Prosecutor Andrew McRae cross examined the forensic psychiatrist, Dickason often shook her head at his questions.

He asked the expert how well she could assess the accused, having only interviewed her via video-link earlier this year, rather than in the immediate aftermath of the incident.

She admitted earlier interviews can often lead to the clearest accounts, but it depends on the person's mental state.

"Being in person may be a little better, but I didn't have any concern about what Lauren was saying to me," Hatters-Friedman also accepted.

McRae suggested that Dickason didn't express to her the true way she felt on the night it happened, contrasting with what she told crown psychiatrists in earlier interviews.

"She reported that she reacted angrily thinking, 'it's never enough' and that she felt 'like a spring getting tighter' and 'I'm going to do something'."

He told the witness that in another interview the defendant acknowledged: "I was worried I was going to give one of the kids a bloody good hiding."

Hatters-Friedman does not believe Dickason's actions were out of anger.

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By Lisa Davies and Laura James