A severely depressed Sydney woman who poisoned her children then tried to kill herself knew what she was doing and can't claim the defence of mental illness, a court has been told.
The woman administered rat poison and an unidentified pink liquid to her three-year-old daughter and four-year-old son, before smothering them and trying to hang herself in the family's Canley Heights home in February last year.
Her distraught husband, whom she had accused of having an affair, returned home to find a trail of blood leading to his dead children.
The woman, who cannot be named for legal reasons, has pleaded not guilty in the NSW Supreme Court to the two murders by reason of mental illness.
In evidence at the trial, which began on Monday, various forensic psychiatrists disagreed over whether her depressive state was so severe she did not know what she was doing.
But in her closing address to the judge-only trial in Sydney, crown prosecutor Margaret Cunneen said the woman's depression was not so extreme it made her incapable of distinguishing right from wrong.
A suicide letter to her parents seeking forgiveness for the killings showed she was still capable of "sophisticated reasoning" and was aware of the gravity of her actions, Ms Cunneen argued.
"There was a clear intent ... to blame the unfaithful husband for the course of action the accused took," she said.
"This is very clearly a case where the accused's ability to judge her actions was substantially impaired.
"(But) she did not have such a defect of reasoning that she was incapable of reasoning at all."
Suicide letters written by the woman have raised questions about her lucidity at the time of killings.
In a note to her husband, the woman blamed his affair for her actions, and claimed to be protecting her son and daughter by taking their lives.
"I have to go away because I don't want to witness the situation in which my husband betrayed me," she wrote.
"I don't want to take away (my children's) lives ... however ... living without love, to me life is just only selfishness and betrayal."
Earlier on Wednesday, forensic psychologist Dr Olav Nielssen told the court the mother's severe depression could be considered "psychotic", though he admitted using the term in a colloquial sense.
"Through her view of the world in a seriously depressed state she saw her life as over, and her actions in taking her childrens' lives therefore right ... because she didn't want them to suffer," Dr Nielssen said.
The woman's lawyer Richard Pontello disputed crown arguments that revenge against her husband was a motive for the killings.
The children's father, whom Ms Cunneen described as a "living victim" of the killings, had bought flowers for a female friend days before the killings.
"There is definitely no assertion of revenge (in the letters)," Mr Pontello said in his closing statement.
"Anger and revenge are very different things.
"In her defence, the accused believed what she was doing was in the best interests of her children."
Justice Clifton Hoeben reserved his decision to a later date, ordering the accused be kept in custody.
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