Barbara Eckersley's state of mind was placed under the spotlight on Tuesday, as she took the witness stand and claimed to have been so confused that she forgot what she had done for days.

The 69-year-old is on trial in the NSW Supreme Court at Goulburn, having pleaded not guilty to the murder of acclaimed former Canberra scientist Dr Mary White.

Mrs Eckersley said on Tuesday that she had felt her mother was suffering and in need of pain relief, but had been "abandoned" by doctors and staff at the Warrigal aged care home in Bundanoon.

She decided to move Dr White to a facility in Coffs Harbour, detailing how she had cried while emailing her siblings about this plan.

Mrs Eckersley also described the toll the situation was taking on her own health, saying she saw Dr White whenever she looked in the mirror.

"Emotionally, it's like watching myself dying slowly," she said in the email.

The jury was on Tuesday shown a video of a bedridden Dr White, who was unable to speak following a major stroke.

In the clip, filmed a short time before her death, the 92-year-old was breathing rapidly and appeared to be agitated.

Mrs Eckersley described how episodes like this had left her feeling increasingly "concerned and distressed" as Dr White's planned move drew nearer, to the point she had "an out-of-body feeling" as she crushed up some sleeping tablets and took them to the Bundanoon facility in August 2018.

Mrs Eckersley told the jury she had only intended to tip half of the powder into her mother's "gloopy" dessert in order to sedate her for the night, but "it all shot out" and she fed Dr White the meal.

She went on to say that she barely slept that night and had "a horrific dream", where she saw and heard Dr White while fending off alien attackers.

Still "haunted" by this the next afternoon, Mrs Eckersley said she suddenly had "a vision" of a barbiturate she had kept from decades earlier, when she had worked as a wildlife carer and used the drug to euthanise animals.

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The 69-year-old said she again felt "disconnected" from herself as she put a small amount of the "green dream" into a container, which she then emptied that evening into her mother's soup.

She told the court that after she began feeding Dr White this meal, the 92-year-old "coughed a couple of times and became unresponsive", prompting her to run for a nurse.

Asked by defence barrister Kieran Ginges why she did not immediately say she had put something in her mother's food, Mrs Eckersley replied: "I had no recollection at all that I had done it."

Dr White died that night.

"I honestly don't think I intended to kill her," Mrs Eckersley said on Tuesday, saying she had only been trying to ease her mother's pain and "relieve her suffering".

Quizzed on whether she felt in the immediate aftermath like she had killed her mother, the 69-year-old responded: "I had no idea I'd had anything to do with it."

Mrs Eckersley said she remembered what she had done with the drugs a few days later, following "a gradual clearing of my fog".

She told the court she went "in a panic" to tell police, while still "confused" and "trying to fill in the gaps about what had happened".

During her interview with detectives, Mrs Eckersley said she had expected her actions to result in Dr White's death.

But on Tuesday, she told the jury she had been "grasping at things to try and explain what had happened", and that she had "allowed the police suggestions to influence what I said".

Mrs Eckersley is set to be cross-examined on Wednesday.

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