A woman accused of murdering her elderly mother told police she felt like she was having an out-of-body experience when she mixed a drug used to euthanise animals into the 92-year-old's dinner and fed it to her.
"It's almost as though I was watching myself," Barbara Eckersley told investigators in a recorded interview played to a jury in the NSW Supreme Court at Goulburn on Wednesday.
"I mean, it's just not something I would do. I can't even kill a fly."
Mrs Eckersley is on trial over the alleged August 2018 murder of her mother, acclaimed former Canberra environmental scientist Dr Mary White, at the Warrigal aged care facility in Bundanoon.
She has pleaded not guilty, with defence barrister Kieran Ginges telling the jury last week that the 69-year-old had not intended to kill her mother, an ailing woman who may have died from other causes.
Mrs Eckersley's defence is set to argue she was just trying to ease her mother's pain and suffering, and that she was "substantially impaired" by a major depressive disorder at the time in question.
A few days after Dr White's death, Mrs Eckersley told detectives she had put a barbiturate drug commonly known as "green dream" into her mother's evening meal.
Mrs Eckersley had obtained the drug, pentobarbital, when she worked as a wildlife carer in Canberra some 20 years earlier.
Asked at one stage what she had thought would happen, she replied: "I guess I was expecting her to die."
Mrs Eckersley told investigators she had not discussed with Dr White the possibility of assisting her to commit suicide.
The court has previously heard Dr White was unable to communicate and was "pretty much confined to the bed" at the aged care facility in the Southern Highlands, having moved there in 2016 after a major stroke left her partially paralysed.
But Mrs Eckersley said her mother had, prior to entering aged care, expressed a desire not to end up "totally out of it" in a nursing home.
"I was just desperate, I suppose," the accused told detectives.
"I just couldn't take it anymore, seeing her suffer."
Towards the end of the interview, Mrs Eckersley said she had been depressed by her mother's ill health, and repeatedly described being "shocked" by her actions.
"I'm just full of astonishment that I did it and I'm just desperately sorry that I did ... it just wasn't rational," she said.
Later on Wednesday, the jury heard from two experts in forensic pathology.
Dr Rebecca Irvine, who conducted the autopsy on Dr White, said a "toxic" amount of pentobarbital was found in the 92-year-old's system.
She reached the conclusion that Dr White had died of "mixed drug (pentobarbital and benzodiazepine) toxicity".
The court has heard that Mrs Eckersley also gave her mother non-prescribed benzodiazepine in the form of crushed sleeping tablets either on the evening in question or the previous day.
The other forensic pathologist, Professor Johan Duflou, said the administration of "green dream" was "likely" to have been a significant factor in Dr White's death.
But Professor Duflou said the 92-year-old may have died of "natural disease processes", or a combination of drug toxicity and other factors.
"To me, it is not clear exactly what she has died of," he told the court.
The trial continues.
